Computer can not start

I had a Dell Dimension 8250. Today, I turned it off and wanted to start it again. But it did not respond and even the power light did not flash. I pushed the power button hard and it still didn't work.I checked the external power cable and found it was OK. The Dell Website says it was a pre-BIOS failure. I don't know what exactly the problem is, does some expert know how to resolve such problem and give me some advice? Thanks a lot!