It might not be power supply unit.
Early this year, I couldn't turn on my PC too, I brought my
motherboard to the service center and they confirm it's ok. The
only thing I figure could be damaged was my display card, because
display cards gets damaged easily because of the heat. I replaced
my display card and everything was alright.
If you have another display card, could try switching around and
try to isolate the faulty device. If your PC was a DIY, suggest you
bring the motherboard to the service center to confirm that it is
working fine. If your display card is the same brand as your
motherboard, you could bring it down for them to test too.
Did you update your bios? If you update your bios and didn't do it
properly, could end up with a non functional motherboard.
Could try to reset the motherboard by shorting jumpers also.
