Plug out the power cable, press the power button a few times to ensure there's no power left, put the power cable back in, and try again.
If you aren't even getting the BIOS screen, it's bad. If the above doesn't work, remove the side of the case, plug out the power cable, press the power button a few times to ensure there's no power left, put the power cable back in, and check do any motherboard lights come on. Turn it on, and again, see if any motherboard lights come on. If none come on, it'll probably be either the PSU or mobo.