By omitting such informatiuon I can't be exact with a troubleshooting article. Here's one to start with. Sorry, I didn't see the OS, so I may be wrong.
I have an HP Pavilion 9900 configurable. It has a 1.2 GHz Athlon processor and 512 MB DDR RAM. I have had it several years, have taken perfect care of it, and have never had a single problem. However, yesterday I turned it on and it it didn't boot past the BIOS screen. Normally I hear the system scrolling through the drives at this point culminating in a system 'beep' and then it moves on to boot Windows. But now it just sits there. If I hit the power button, instead of turning off it immediately restarts and does the exact same thing.
I have verified the hard drive is not bad. All my cables are plugged in correctly. My suspicion is that the problem is the motherboard or memory. Does anyone have any idea what it could be? I had just had my computer on ten minutes before and had no problems. This doesn't make any sense.