user60959: can someone tell me why my L led on arduino uno glowing continuously I tried restarting my pc, changing USB port 
I also tried uploading an empty file and resetting the  Arduino board but nothing seems to work. Does it mean I have to buy a new one?

user16780: Don't let that pin float undefined: If undefined (INPUT), pull it to GND to have the led OFF.
If ever possible, try to use the LED_BUILTIN pin as an OUTPUT pin.
And signal some information there. On an UNO (or other 328-based Arduino), this hint might collide with the SPI_SCK pin, which is D13 (PB5 on an atmega328). But as an SPI Master, this pin is at least OUTPUT (and thus will signal SPI activity).

60959: <p>can someone tell me why my L led on arduino uno glowing continuously I tried restarting my pc, changing USB port 
I also tried uploading an empty file and resetting the  Arduino board but nothing seems to work. Does it mean I have to buy a new one?</p>
 16780: <p>Don't let that pin float undefined: If undefined (INPUT), pull it to GND to have the led OFF.</p>

<p>If ever possible, try to use the LED_BUILTIN pin as an OUTPUT pin.</p>

<p>And signal some information there. On an UNO (or other 328-based Arduino), this hint might collide with the SPI_SCK pin, which is D13 (PB5 on an atmega328). But as an SPI Master, this pin is at least OUTPUT (and thus will signal SPI activity).</p>
