|I know there are issues with ROM corrupting if you leave a computer off for a while |
Is there. Strange that in twenty five years I have never know a ROM to get corrupted through being left off. I have powered up computers that have been idle for years without a problem
There is a possibility of the CMOS memory getting corrupted because the CMOS battery has died but I don't think that is you problem here. If that were the case you would at least get something on the screen even if it was CMOS Checksum Error.
I would first open the case and reseat all the memory, expansion cards and connections and see if that work. These things can creep, especially if the computer had been moved about.
If that fails I would start looking at the power supply. The fact that fans spin and disk drive spin does not mean that the power supply is functioning correctly.