if its an old pc. either your PSU or your motherboard is failing. if its an old type of PSU, make sure teh voltage switch is properly set to either 110v/240v. you can also try reseating your RAM sticks.
if this is a new build, recheck your PSU power cables (both the 24-pin and the 6-pin) are properly connected to your motherboard.
if you havent used this pc for awhile, oxidation on your hardware most likely is preventing your system to start up. shutdown. turn off psu. unplug power chord. you can open your case. reseat RAM and GPU. press the power button for 1 minute. put everything back on. and try to boot it.