When you say "mega2560", do you mean an Arduino Mega 2560 type of board (doesn't matter if it's a clone) or do you mean just an ATmega2560 chip? What I'm wondering is if there is a crystal/oscillator connected to the ATmega2560. If the fuses on the ATmega2560 have been set to use an external clock source then you need to have an external clock source attached, even if only to change the fuses back to use the internal oscillator.