Desktop>Laptop?
Desktop.
Turn the computer off and disconnect it from the power
Take off the side of the case and touch the bare metal chassis to discharge static.
Remove Ram..Video cards from their slots
Plug in the RAM [one stick at a time].
Also try different slots.
Ensure it is seated correctly.
Reseat Video card.
Check Hard drive and Motherboard connections [IDE]
Take them off and reseat them.
Inspect the case for dust etc.
Remove the dust..do not use a vacuum cleaner
Check that the processor fan rotate. Check for dust.
Turn on and listen for a beep.
A properly working computer should beep once while booting, and the lights on the keyboard should flash while booting.