I wanted to add my voice to this chorus. Same exact problem. Using Windows 7 + Uno. I was able to upload the blink sketch a couple months ago. Didn't touch anything arduino-wise for a couple of months, I go back to play around and upload a new sketch, suddenly Windows no longer sees the Uno at all, not even as an unknown device. Uno is still getting power. I've googled around and can't find an answer anywhere. Tried using different cables and uninstalling/reinstalling the IDE to no avail.