Try clearing the computer of power. Put your psu back in make sure everything is plugged in correctly, also on the back of the psu there might be a little red switch to change voltage try screwing with that but anyway if not take out the cmos battery (the tiny circle battery on board) unplug the power and hold your power button for like 5-10 seconds. Then wait for 10 mins, plug it back in and try to start.

chances are the psu is dead, you can buy a decent one at like staples or something for like 50 bucks( if it doesnt fix it return it), then you know its the mobo

Also who know it could be a bad switch on the case itself, you can try finding the powerleads on the board and crossing them but if your not that computer literit i dont suggest cus you might screw something else up