USB not showing up

I've been using the arduino uno for a while, and it stopped working today. I had a sketch running that read input from a 2.4ghz Rx and sent the signal over serial (via usb). It had been working fine, so I put it down to go set up a circuit for the rest of my project. When I reconnected the arduino, the sketch was no longer running (or at least Tx light stopped flashing) and it would no longer show up over USB. I've tried on two computers with two usb cables, so thats not the problem. I hope the mega8u2 isn't fried, but I'm not sure how to check if it is.

Edit: The power light turns on, hitting reset makes pin 13 / 'L' flash, and the power outputs (3.3v and 5v) work.

I have a mega2560 that I was playing with serial on and it similarly is no longer detected, post here,58243.0.html I'll let you know if I figure anything out, it seems to be an 8u2 problem.