re-boot PC ?
Start by removing everything connected to the arduino then try this:Take these steps: Power off the board completely (remove the USB cable). Hold down the Reset button, keep it held down (or, run a jumper wire from the RESET pin to the GND pin). Still holding down Reset, reconnect the USB cable. Start uploading a sketch that does not have this problem (I use the Blink sketch). I have seen this happen several times when I flood the serial output. An indication is the tx light is on. My quick fix is to add a 5 second delay in the setup then I can reset and gain control before the serial port floods. Secret do not keep the console constantly busy printing. Good Luck & Have Fun!Gil
Try on another PC ?