Can you get a basic video card to test with - even basic PCI? Your Maxtor 80G will likely be misdetected on the HPT366 controller. Might want to go shopping for a Promise ATA100 card ($25-30).
Most likely it was dirt on one of the contacts between the RAM module and slot, as DD suggested. Pulling the RAM sticks several times cleaned the contacts enough for it to work.
Pierrot, there is often no clearcut explanation when a system goes from working to not working (or the other way). InactiveX was probably on the mark, Often, the mere physical handling of the internal components can cause a change from booting to no-boot & back again. I've had systems that work fine with the case disassembled & horizontal. When I reassemble & stand it up it won't boot. Usually means some stress to the mainboard is making an existing defect go over the edge.
Glad you're back in business now.