I have two Arduino Uno Boards and I cannot upload any sketches to them. Both of these boards were, and appear to still be, working. I have uploaded sketches to both of them in the past and they still run the sketches that they last had uploaded to them but I cannot upload any new sketch, including Blink. The on-line IDE gets as far as saying BUSY but never uploads the sketch.
I have tried the following. Uploading from three different computers. Three different USB cables. Two different Arduino Uno boards. Many computer reboots.
Any ideas of how to solve or troubleshoot this problem would be greatly appreciated.