Trying to upload to a Uno - (may be a clone) , r5 lights, rx blinks, then nothing. I've tried 2 cables, rebooted PC, same issue. It was working fine for an hour or two. Uploading to a duemilanove works fine.
Are you sure that the right board is selected in the Tools menu?
Are you sure that the right serial port is selected in the Tools menu? In Windows with the clones, the serial port can change.
Have you tried followed the instructions in the upload troubleshooting page?
Ah, I've discovered they work when no pins are attached, so I've plugged something in the wrong hole doh!
Pins 0 and 1 on the Uno are used for communication with the computer, including uploads. So you should avoid connecting anything to those pins unless you have a specific reason for using them.