First, remove the motherboard battery (if present) and try a double-boot:  leave it powered on for about half an hour, then very quickly turn it off and on again.  Some Macs will refuse to boot without a working PRAM battery, but this trick can sometimes get a little juice into the PRAM circuit and allow it to boot.

If it works, you need a new PRAM battery - 1/2AA 3.6V