What is the project that you're working on? Anything connected to pins 0 and 1? What else was connected when it failed the first time? Did you ever manage to upload to this board?
You can run the Loop-Back Test Instructions. If it fails, we known that the problem lays with the 16U2. If it passes, the problem is more than likely with the 2560 processor.
When you press and release the reset button, does the L-LED flash a couple of times? If not, the 2560 does not have a bootloader or is damaged.