Pc doesn't boot; fan works

I was using the desktop in question when it automatically shut down and it hasn't successfully turned on since then.

We changed the power supply, but it still doesn't boot. The screen turns on then turns off so fast and shows the words "Power saving mode". (That normally just happens before after turning off the pc)

The fan in the CPU works and there's light for DVD ROM when I push the turn on button of the CPU.

What do you think is the problem of this CPU and how can I solve it?

Thanks a lot.

Cheers