user37598: is it my coding wrong or my board have problem because my lcd does not display anything. 
here my coding
#include <Wire.h>
#include <LiquidCrystal_I2C.h>


// set the LCD address to 0x27 for a 20 chars 4 line display
// Set the pins on the I2C chip used for LCD connections:
//                    addr, en,rw,rs,d4,d5,d6,d7,bl,blpol
LiquidCrystal_I2C lcd(0x27, 6, 2, 3, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C add 27 / 3F



void setup()   /*----( SETUP: RUNS ONCE )----*/
{
  Serial.begin(9600); //
  lcd.begin(16,2);         // initialize the lcd for 16 chars 2 lines, turn on backlight
  lcd.backlight();

}

void loop()   /*----( LOOP: RUNS CONSTANTLY )----*/
{

  lcd.setCursor(0, 0);               
  lcd.print("HELLO");

  delay(200);

  lcd.setCursor(0, 1);            
  lcd.print ("HI");

  delay(2000);

  lcd.clear();

  delay(2000);

  lcd.setCursor(0, 0);               
  lcd.print("NICE");

  delay(200);

  lcd.setCursor(0, 1);            
  lcd.print ("GOOD ");

  delay(2000);

  lcd.clear();

  delay(2000);
}


this is how i wirring it. what should i do?

user30005: the black bars indicate that the lcd isn't correctly initialized.
check the hardware / connection and make sure that your software initializes it correctly.

user-1: Well, I have the hardware, so I tested OP's configuration. It didn't work for me either. Same OP's problem.
The cause is the I2C address we are using (0x27). That's not my LCD address, probably neither your address. It's 0x3F in my case.
I discovered it I2CScanner, a short sketch that lists all I2C devices connected at that moment. So, run it (with the LCD connected) and watch with Serial Monitor for the address of your LCD.
Then replace all references to 0x27 (like lcd(0x27, 16, 2)) with your real address. Just change one value for the other.

This is I2CScanner
//
// Version 1
//    This program (or code that looks like it)
//    can be found in many places.
//    For example on the Arduino.cc forum.
//    The original author is not know.
// Version 2, Juni 2012, Using Arduino 1.0.1
//     Adapted to be as simple as possible by Arduino.cc user Krodal
// Version 3, Feb 26  2013
//    V3 by louarnold
// Version 4, March 3, 2013, Using Arduino 1.0.3
//    by Arduino.cc user Krodal.
//    Changes by louarnold removed.
//    Scanning addresses changed from 0...127 to 1...119,
//    according to the i2c scanner by Nick Gammon
//    http://www.gammon.com.au/forum/?id=10896
// Version 5, March 28, 2013
//    As version 4, but address scans now to 127.
//    A sensor seems to use address 120.
// Version 6, November 27, 2015.
//    Added waiting for the Leonardo serial communication.
// 
//
// This sketch tests the standard 7-bit addresses
// Devices with higher bit address might not be seen properly.
//

#include <Wire.h>


void setup()
{
  Wire.begin();

  Serial.begin(9600);
  while (!Serial);             // Leonardo: wait for serial monitor
  Serial.println("\nI2C Scanner");
}


void loop()
{
  byte error, address;
  int nDevices;

  Serial.println("Scanning...");

  nDevices = 0;
  for(address = 1; address < 127; address++ ) 
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");

      nDevices++;
    }
    else if (error==4) 
    {
      Serial.print("Unknown error at address 0x");
      if (address<16) 
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);           // wait 5 seconds for next scan
}


37598: <p>is it my coding wrong or my board have problem because my lcd does not display anything. 
here my coding</p>

<pre><code>#include &lt;Wire.h&gt;
#include &lt;LiquidCrystal_I2C.h&gt;


// set the LCD address to 0x27 for a 20 chars 4 line display
// Set the pins on the I2C chip used for LCD connections:
//                    addr, en,rw,rs,d4,d5,d6,d7,bl,blpol
LiquidCrystal_I2C lcd(0x27, 6, 2, 3, 4, 5, 6, 7, 3, POSITIVE);  // Set the LCD I2C add 27 / 3F



void setup()   /*----( SETUP: RUNS ONCE )----*/
{
  Serial.begin(9600); //
  lcd.begin(16,2);         // initialize the lcd for 16 chars 2 lines, turn on backlight
  lcd.backlight();

}

void loop()   /*----( LOOP: RUNS CONSTANTLY )----*/
{

  lcd.setCursor(0, 0);               
  lcd.print("HELLO");

  delay(200);

  lcd.setCursor(0, 1);            
  lcd.print ("HI");

  delay(2000);

  lcd.clear();

  delay(2000);

  lcd.setCursor(0, 0);               
  lcd.print("NICE");

  delay(200);

  lcd.setCursor(0, 1);            
  lcd.print ("GOOD ");

  delay(2000);

  lcd.clear();

  delay(2000);
}
</code></pre>

<p><a href="https://i.stack.imgur.com/xwUC4.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xwUC4.jpg" alt="enter image description here"></a></p>

<p>this is how i wirring it. what should i do?</p>
 30005: <p>the black bars indicate that the lcd isn't correctly initialized.</p>

<p>check the hardware / connection and make sure that your software initializes it correctly.</p>
 -1: <p>Well, I have the hardware, so I tested OP's configuration. It didn't work for me either. Same OP's problem.</p>

<p>The cause is the I2C address we are using (0x27). That's not my LCD address, probably neither your address. It's 0x3F in my case.</p>

<p>I discovered it I2CScanner, a short sketch that lists all I2C devices connected at that moment. So, run it (with the LCD connected) and watch with Serial Monitor for the address of your LCD.</p>

<p>Then replace all references to 0x27 (like <code>lcd(0x27, 16, 2)</code>) with your real address. Just change one value for the other.</p>

<p><a href="https://i.stack.imgur.com/QTPV9.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QTPV9.jpg" alt="Hello World"></a></p>

<p>This is I2CScanner</p>

<pre><code>//
// Version 1
//    This program (or code that looks like it)
//    can be found in many places.
//    For example on the Arduino.cc forum.
//    The original author is not know.
// Version 2, Juni 2012, Using Arduino 1.0.1
//     Adapted to be as simple as possible by Arduino.cc user Krodal
// Version 3, Feb 26  2013
//    V3 by louarnold
// Version 4, March 3, 2013, Using Arduino 1.0.3
//    by Arduino.cc user Krodal.
//    Changes by louarnold removed.
//    Scanning addresses changed from 0...127 to 1...119,
//    according to the i2c scanner by Nick Gammon
//    http://www.gammon.com.au/forum/?id=10896
// Version 5, March 28, 2013
//    As version 4, but address scans now to 127.
//    A sensor seems to use address 120.
// Version 6, November 27, 2015.
//    Added waiting for the Leonardo serial communication.
// 
//
// This sketch tests the standard 7-bit addresses
// Devices with higher bit address might not be seen properly.
//

#include &lt;Wire.h&gt;


void setup()
{
  Wire.begin();

  Serial.begin(9600);
  while (!Serial);             // Leonardo: wait for serial monitor
  Serial.println("\nI2C Scanner");
}


void loop()
{
  byte error, address;
  int nDevices;

  Serial.println("Scanning...");

  nDevices = 0;
  for(address = 1; address &lt; 127; address++ ) 
  {
    // The i2c_scanner uses the return value of
    // the Write.endTransmisstion to see if
    // a device did acknowledge to the address.
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0)
    {
      Serial.print("I2C device found at address 0x");
      if (address&lt;16) 
        Serial.print("0");
      Serial.print(address,HEX);
      Serial.println("  !");

      nDevices++;
    }
    else if (error==4) 
    {
      Serial.print("Unknown error at address 0x");
      if (address&lt;16) 
        Serial.print("0");
      Serial.println(address,HEX);
    }    
  }
  if (nDevices == 0)
    Serial.println("No I2C devices found\n");
  else
    Serial.println("done\n");

  delay(5000);           // wait 5 seconds for next scan
}
</code></pre>
