Three probablities come to mind:
If you can get to the BIOS Setup screen or can boot from CD or floppy:
1) There was a software crash that wrote garbage to the HDD boot area.
If you want to recover data, remove the HDD and connect it to another machine for retrieval. Or just re-install the operating system.
2) It really went into hibernation and your new BIOS has a problem.
Try rolling back to the original BIOS. This may require physicall replacing the BIOS ROM on the motherboard.
If you canNOT get to BIOS Setup or boot from either CD or floppy:
Try 2), above; but be prepared to accept 3), below.
3) The CPU (or something else) melted from overheating.
Follow 1), above. If you can do the work yourself, it MIGHT be worthwhile to start replacing MoBo parts. If you have to pay a shop for repairs it's probably cheaper to start over with a new computer.