I will tell you the story behind the problem.
The OS needed to be re installed. It has 2 HDDs so I had to un-plug one to see which is which, to save as much data as possible. After unplugging the first one, it booted with the installation CD.
It wasn't the HDD I was looking for so I turned it off and unplugged the other one. After unplugging and turning on, right after booting test, the HDD was turned off and the only thing that was working was the CPU fan. I turned it off the switch on the power supply, holding the power button didn't do it.
Now it doesn't turn on anymore. I tried with another test power supply but the result was the same, it won't turn on. Any ideas?