First I would disconnect the power cable from the PSU to the wall outlet, then disconnect all power cables from the PSU to the motherboard, drives, GPU, etc and once everything is disconnected press the power button like 10 times then hold it in for about 30 seconds then let it go. After that plug everything back in and see if you can start it up....
What are your system specs? Does your motherboard have a ps/2 port a keyboard? (Circular purple port with about 6 holes in it) And do you have a ps/2 keyboard?