Sounds like your hard drive might've given up the ghost, although you should probably go through and check all the obvious stuff - connections, termination, cables, etc. Any clicking or unusual noises from the drive? Battery leakage on the motherboard that could've affected anything? Lose your NVRAM settings that contain some boot information? Does the drive show up in early startup? Detect it from HDToolbox if you boot from floppy? Etc.
If I understand correctly, from your previous post it sounds like you already have 3.1 ROM's installed?
