Is it a 400/500/550/667 or newer?
Remove all power, battery, accessories.
Press the reset button for 5 seconds
(on 400/500/550/667 it is next to the modem on the back. On the newer ones, it is under the Eject key on the logic board.)
Reattach battery and power. Power On, let sit for several minutes. If it doesn't work then, try removing ram and using only one stick at a time.
TEG