First check: working without RAM and Display card (only Power supply + Motherboard + CPU). listen to speaker on mortherboard. If have beep, motherboard and CPU work good. If not, maybe motherboard have problem, or CPU die.
Second check: clean RAM and display card by Eraser, then take in. Turn on power. If not work, change Ram. If Ram work good, change Display card.
third check: Change Power supply.
Good luck !