Just one thought: Did you get out your mobo's BIOS buffer battery (if there's one and not a soldered accu) for some time and then tried it again with the insert key?
I had the same effect you mentioned after overclocking my Duron 650 too much (MSI K7T Pro2A mobo). It seemed completely dead, but after setting the BIOS back to defaults by stealing the battery it was all up again and I was able to start over.
Only 2 cents but you never know...
Good luck!
Beagle