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My Arduino 2560 Power on, Won't upload

I just bought an Arduino 2560 Board, I was using it with windows 7 and it was working perfectly.

The problem is that now the Arduino is not detected by any of the operative systems, I am currently testing with Mac Snow Leopard, Linux and windows 7. The reset botton is working, and when I plug it to the computer the Power LED turns on......

Any suggestions on how to fix this? 

 

THANKS!