Re-bootload it, and then upload the bare minimum sketch. Does that upload work?
If yes (and assuming subsequent upload attempts still fail) - you'll notice that when you press reset, the light blinks a few times. This is done by the bootloader to let you know that the bootloader is running. When you're trying and failing to upload, do you get the blink pattern then too? (this verifies that autoreset is working).
Is anything connected to pins 0 or 1, or to reset? Anything connected to those pins is likely to interfere with uploads.
Is anything connected to it at all?