Assuming it worked before, remove the bios battery, wait 10 minutes or so. Then leave only 1 HDD and Ram plugged in, as well as GPU.
Try again. If it works, shut it down, reconnect a new hardware piece and and boot, entering setup. Repeat until you have plugged all you hardware.
If it fails somewhere during this process, its your GPU (or a faulty drive\HW piece).
Sorry for the generic answer, but I tried to work with the info you provided. 😉