Can't upload to my Arduino UNO
First of all DON'T PANIC, it's more than probable your Arduino is not broken.
Start running the following checks:


is any other software using the same serial port? serial ports cannot be concurrently opened by multiple processes...
disconnect the board, reset your PC, wait until the boot process is completed and then connect again: does your OS recognize the COM port? (Windows emits a sound, GNU/Linux logs a few lines...)
check the COM port matches, may be your board is no more recognized as COM3 but as COM4, depending on multiple factors
if you have anything wired to your board, disconnect everything, unplug the board and then re-connect the board alone, with nothing connected to its pins: you might have a short somewhere or using pins 0 or 1 (marked as RX &amp; TX, they are also used for communication)
is your USB cable connection flaking? it might seem stupid, but if you have multiple cables running around your PC it is indeed possible to pick the wrong one or loose it from the socket...
are you using a cheap cinese clone? There are clones using counterfeited USB chips which get banned by Windows automatic updates, but a workaround exists even for that


While it is possible to brick the Arduino boards via software, there are very little possibilities you did it because it involves using tools completely hidden to the Arduino occasional user / newcomer.