Try booting with a minimal hardware setup:
You want to boot only with your cpu and heat sink, PS2 keyboard, video card and only 1 memory module. That's it.
If you have a case speaker connected to the motherboard, leave it connected.
If needed, try booting with each bios chip.
Make sure the your power supply is disconnected any time you remove or install any hardware.
I would remove the cmos battery before installing or replacing the bios chip.
Are there any beep codes from the speaker?
Do any fans spin up when you power on your system?
Do you have another psu to test with? It's possible that there was a "coincidental" power supply failure.
If all else fails, remove the mobo battery and leave the clear cmos pins jumpered for 12 - 24 hours.
Sometimes you need to clear the cmos for a long time.