So you've tried the following?
- take out hard drive, floppy, cd-rom, battery and tried it on AC
- hold down power button for up to 20 secs until LED lights go off
If so, try this:
1) tighten all screws underneath the laptop (the center one acts as a grounding screw and must be securely in)
2) with everything out of the system (peripherals, HD, etc.) and power disconnected, hold the power button down for 30 secs which forces an internal reset
3) reseat the RAM