avrdude: ser_open(): can't open device "\\.\COM3"? My computer cannot detect my Arduino Uno anymore,

Reset button- &gt; L led light blinks and it turns on forever
Arduino 1.6.5: Tools-&gt;Port (greyed out) I remembered that previously it wasn't like this
Unable to upload any sketch codes into the Uno anymore
The existing code uploaded onto the Uno is FastLED(FirstLight) and it will work on my LEDs when I plugged them in

I have tried to .... but none fix my issue

Restart my computer
Press the reset button on the Uno
Using the device mananger to change COM ports
Device manager -&gt; Other devices -&gt; (nothing about Arduino Uno is found here)
Plugged Uno into another computer but still doesn't work
Reinstalled IDE

Problem shown in sketch,

avrdude: ser_open(): can't open device &quot;\.\COM3&quot;: The system cannot find the file specified.
Problem uploading to board.  See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.
I have experienced a similar issue after plugging in another Arduino device. I didn't set up my environment for a long time and thus forgot he iterates the COM-Index. Maybe this is too easy, but you didn't provide much information.

This solved my problem:
Open Arduino application > Tools > Port > Change to the target Port

If you are unsure which port to choose from: (on Windows)
Open Device Manager > Connections (COM &amp; LPT) > Usb Serial Port (COM[x])
where "[x]" is the index you are looking for.