So it's dead, but you're getting 5v on the Z80? Did you check the board for cold solder joints (especially at the power headers), or broken traces? That should be one of the first things you look at.
Also, what's pin 26 (reset) doing on the Z80? If it's toggling every few seconds, then your board is stuck in watchdog and I would test the CPU board ROMS in a programmer.