My PC did that a few weeks ago I had to get a new PSU, that's prob 90% it, but if its an older computer there's a chance a capacitor on your motherboard blew, check your motherboard info somehow, online or whatnot see if it was made with solid capacitors, if so then you got a dead PSU, or you could check the motherboard yourself, may need a flashlight, just look at the capacitors for any of them being blown, bulging or leaking fluid out the top or bottom.