System model? Age? Define "not working" -- won't powerup at all?
In that case, unplug, remove or disconnect the battery, and hold the power button for 30 sec. Remove and reinstall the memory and hard drive. Try powering up on AC only - if it will, but won't with the battery installed, the battery is bad.
If it still won't power up on AC only after the reset above, the mainboard is likely bad.