|First pull ac power and then press power on button a few times. Then replace ac plug and see if it boots correctly.|
If not then use bios to see if there is a hard drive. Some have a sort of autodetect selection.
If still fails then get the hard drive diagnostic for that brand. Almost every hard drive company offers great software that can run usually from a cd or dvd drive. You boot to the cd.
Then maybe I'd go in the computer (after you remove ac and press power button a few times and use esd strap) to re-seat hard drive cables. Look for any wear or damage on the cables. Test the power supply maybe too.
A Pit Bull is like a gun you can pet. And there is no safety on it.