8400 after power outage

First suggestion, check the AC power receptacle or power strip where you have it plugged in to verify there is AC power there.  Could be a house circuit breaker has tripped.

If the AC power is OK, unplug the AC power cord from the PC (Power Supply) for about 1 minute and then reconnect.  There is a "crowbar" circuit in the power supply - basically an electronic circuit breaker and if it happened to trip you need to remove the AC power from the power supply input.

If this does not fix it, check the motherboard to see if there is a motherbord power light that is lit.  The color of the light varies from model to model and the color isn't important either it will be on or off.  If it's off most likely the power supply is shot and will have to be replaced.  However, unfortunately, if it's on it does not mean the power supply is good.   There is another procedure to see if the power supply will turn on, but again it doesn't denote a good power supply.  Run through the checks I've listed and post back with results before we go further, or if you don't feel you can do more technical work it's time to take it to a repair shop.

Also try this--remove all internal cards--network, phone modem-if you have one, sound card etc and leave only the video card. If you can boot, reinstall each card and reboot after each one until you find the one that failed. Network, sound and phone modem cards are very sensitive to power surges. Plug the computer directly into a good outlet and skip any power strips or UPS units.

I have a green light on the motherboard but no lights any where else. Not around power switch or 4 diagnostic lights on rear.