Dell Dimension 4550 wont turn on

Anything showing on the diagnostic LEDs on the back panel (ABCD, amber/green)?

Are those LEDs on the motherboard? if so, then there is one on that's green. otherwise, there doesnt seem to be any lights on anywhere else.

On the back of the system near the printer port, there should be four diagnostic LEDs.

oh. I see those, but none are on.

First thing I'd try is removing all the PCI cards, then see if the system will power on. 

If it comes up, try adding cards back one at a time & find the bad card. 

If not, then it is either the power supply (Just because the MB LED is on, doesn't mean the P/S is good) or the motherboard.

If under warranty, call 1-800-624-9896  if not get a new power supply  1-800-372-3355 is Dell Spare parts

thanks for the help. I took out all the PCI cards like you said, and it still doesnt turn on. Im pretty sure the warranty ran out about a month ago, so it seems im gonna need either a new PS, or a new motherboard.