Computer shut off

Hello,
I cleaned up my computer inside, you know, all hardwere with the special sry. I nomally do this once every 6 months, just to keep it clean.
This time, I take out the power supply and the fan thats keep cool the processesor instaled on the mother board.
After the cleaning and put evrything back in place, I restart my computer and just afrter bios finish and before windows load it shut off by it self.

What could be the problem...?
What can I do to solve it...?
Do you think it has to be the the power supply or the prossesor's fan...?

Thanks