Is the power LED lighting up orange? If so, it most likely is a power supply issue. Sometimes, by unplugging, waiting for about 30 seconds, then replugging the issue has corrected itself on some old dells here. Disconnected the 24pin connector from the motherboard then plugging it back in has also sparked life back into these systems.
nobus
try with the minimum : mobo + cpu + 1 ram stick, video card, PS --> any display? if NO it is one of the connected, swap ram, video card, PS ; leaves only mobo or cpu
if Yes, add devices till the problem appears
***it can alkso be due to bad caps in mobo or PS : www.badcaps.net