Dell Optiplex Gx1 PII processor

I have a problem with the above Machine model.
It was workinf fine conected to the net through another machine. One day i switched it on and could not swith on at all. when i open, there is power entering to the board. I changed the processor, ram power surply still it cant boot. i have transfred its powersurply to another machine and it worked fine. What could be the problem?