Arduino Uno No Longer Detected by Computer

After trying to upload a program to my Uno, neither my Mac (OSX 10.7) nor my PC (Win7) will even detect my board; it does not even show up in device manager on windows. When connected via USB the green led turns on and the blink program that I loaded onto it last still runs, but there is no way to see the board on either computer. The drivers were installed correctly because i have been able to upload programs before, and even if i wanted to re-install the drivers, the board doesn't show up in device manager. i have no idea where to go from here, so any ideas of how to fix this are welcome.

Plug your Arduino UNO into your computer then re-boot the computer and see what happens. There is a problem with the Atmel USB ID and USB keyboards and mice.

Bob Macklin
K5MYJ
Seattle, Wa