have you located cmos reset jumper (if you removed battery you may need to wait a long time to discharge capacitor)
have you checked voltages 5V & 12V on a spare molex connector black = 0 = ground, yellow = 12V red = 5V
On Intel CPU ATX 12 V to motherboard is critical to start up
Have you checked capacitors near to CPU (part of regulator circuit) for bulges or leaking ?
(bad capacitors needs expert to replace or new motherboard)