My guess would be something internal has died and the BIOS POST is crashing because of it. Try getting into the BIOS (via F2 or Delete as the system starts up) and see if that works. If it doesn't, well, I'm assuming you've got some tech skills for the next bit:
Remove all unneccessary internal components (Hard Disk, CD-ROM drives, etc, as well as any PCI cards you may have), and then boot the PC.
If it works then replace them one by one, booting in between to see if the system still boots. Once you find the hardware that stops it booting then you'll need to replace that bit.
If it doesn't work then the motherboard is likely foobar-d.
If you don't have the tech skills, you'll want to get someone who does to take a look at it if you got this far.
(And I warn you, it may be expensive if they're not your mate... maybe even if they are !
)