I assume you are talking Desktop with the factory installed original PSU.
What brand and model of computer, how old is it, and what peripheral devices do you have hooked up to it (including usb ports in use)?
Have you added extra drives or graphics cards?
Does the machine just shut down or does it reboot itself?
Fortunately, PSU's are neither expensive or hard to replace, more so than a mobo, which could also be the problem.