user39742: I am not sure why the ON LED and the L LED keeps blinking on my Arduino.
I was just trying to implement a simple circuit using breadboard, and everything worked fine until I unplugged one wire with the power turned on. I accidentally made the wire touch something in the Arduino, and the LED in the breadboard became extremely bright. Then the ON LED and the L LED in the Arduino started to keep blinking, and I cannot download code to Arduino anymore. I am using IAR and IAR says AT91SAM3 reset: Communication breakdown after resetting target. Abort debug session?.
I think maybe there is a short cut when the wire touched the Arduino... But how can I determine what the problem is? Thank you very much, I am totally new to Arduino and has very limited knowledge about hardware...

user31105: You should split your problem in pieces. Like you cannot get communication with your Arduino again, so check first:

If you plug in the Arduino / USB, do you get a message on your computer?
Is it visible in the systems manager?
Is it visible by the IDE?
If you connect an adapter (12 or 9V), does it make a difference?
If you try another Arduino, does it work like it should be?

If you are a beginner, always make sure:

Never change any wiring with the Arduino on
Never have dangling wires, if you do not need a wire for the time being, either remove it completely or put it in a breadboard hole that is not used
Never change components with the Arduino on
After rewiring/adding/changing components, double check first

Update for comment below because it does not fit as comment:
To buy a cheap/fake (but working) Arduino uno, e.g. 
Arduino Uno from Aliexpress

39742: <p>I am not sure why the <code>ON</code> LED and the <code>L</code> LED keeps blinking on my Arduino.</p>

<p>I was just trying to implement a simple circuit using breadboard, and everything worked fine until I unplugged one wire with the power turned on. I accidentally made the wire touch something in the Arduino, and the LED in the breadboard became extremely bright. Then the <code>ON</code> LED and the <code>L</code> LED in the Arduino started to keep blinking, and I cannot download code to Arduino anymore. I am using IAR and IAR says <code>AT91SAM3 reset: Communication breakdown after resetting target. Abort debug session?</code>.</p>

<p>I think maybe there is a short cut when the wire touched the Arduino... But how can I determine what the problem is? Thank you very much, I am totally new to Arduino and has very limited knowledge about hardware...</p>
 31105: <p>You should split your problem in pieces. Like you cannot get communication with your Arduino again, so check first:</p>

<ul>
<li>If you plug in the Arduino / USB, do you get a message on your computer?</li>
<li>Is it visible in the systems manager?</li>
<li>Is it visible by the IDE?</li>
<li>If you connect an adapter (12 or 9V), does it make a difference?</li>
<li>If you try another Arduino, does it work like it should be?</li>
</ul>

<p>If you are a beginner, always make sure:</p>

<ul>
<li>Never change any wiring with the Arduino on</li>
<li>Never have dangling wires, if you do not need a wire for the time being, either remove it completely or put it in a breadboard hole that is not used</li>
<li>Never change components with the Arduino on</li>
<li>After rewiring/adding/changing components, double check first</li>
</ul>

<p>Update for comment below because it does not fit as comment:
To buy a cheap/fake (but working) Arduino uno, e.g. 
<a href="https://www.aliexpress.com/item/Free-shipping-UNO-R3-MEGA328P-with-usb-cable-for-Arduino-Compatible-Dropshipping/32241237838.html?spm=2114.search0104.3.17.u1gbjG&amp;ws_ab_test=searchweb0_0,searchweb201602_4_10152_10065_10151_10344_10068_10345_10342_10343_10340_10341_10541_10562_10084_10083_10304_10307_10301_10177_10060_10155_5680017_10154_10056_10055_10539_10537_10312_10536_10059_10313_10314_10534_10533_100031_10103_10073_10102_10557_10142_10107,searchweb201603_25,ppcSwitch_7&amp;btsid=74489aca-62ba-4565-b712-838d7fb5f5d1&amp;algo_expid=4aeaf1be-d6ec-4fc5-8988-7534ccd34d9b-2&amp;algo_pvid=4aeaf1be-d6ec-4fc5-8988-7534ccd34d9b" rel="nofollow noreferrer">Arduino Uno from Aliexpress</a></p>
