Some things to try
1.Remove all power to machine by removing power cord and battery
Press and hold the power button for 30 seconds
reconnect mains lead (leave battery out) and attempt to reboot.
Remove and refit memory and retry to boot
Remove and refit hard drive and retry to boot
If still no good do 1. again and alsoremove the CMOS battery if you can get at it. before doing 2. and 3.