Arduino Uno cannot upload any sketch: avrdude: stk500_recv(): programmer is not responding
First stop for me is always the connections (which you've done), the sketch itself (which doesn't sound likely) and then the COM port / baud rate.  If your sketch involves anything with the serial connection, ISR's or power supplies, that could be an issue with the Arduino not listening.  It sounds like you've covered this as well by trying simple sketches.  Next, check the COM port on the host computer (sometimes it changes in Windows) and inside the IDE, the baud rate, and open the IDE serial port and wander through baud rate settings until you get something legible as a response in the serial window.  Also, double check inside the IDE that you still have the correct board selected.  Occasionally / rarely, I've had to reboot Windows to cure the problem.

The good news is, this pain is significant enough that you don't usually make the mistake again, and will probably never forget how you solved it....