URGENT: Computer will not switch on

Hi Guys,

I have a desktop at home, and I opened it to look what the label on the RAM said. I closed it and tried to turn it on, but when I pressed the button, the fan goes on for a split second and then goes off and turns off.
I didn’t take anything out. If it helps, we had a 2 hour powercut last night and this is the first attempt at switching it on since.

It’s also real dusty in there too.

Thanks
Benji