It could be PSU, or GPU or MB. First try on-board graphics(VGA) by removing the GPU from the system, see if you can get display. If you get the display, then it may be the GPU is faulty. If you don't get the display then it could be PSU or MB is faulty.
Second try another PSU to test.
Third take out all the RAM sticks and start the PC, if it gives beeping sound then the MB is ok, if not the MB is dead.
Fourth try one RAM at a time, so you can isolate bad RAM if any.