My brother's pc wont turn on!

If the power has been going on and off, and it is a cheaper quality PSU(even a good one could), there is definitely a chance it has gone south.

There is a chance (slight but still a chance) that the actual power button is messed up. Since you know where the power button hooks into the motherboard, remove the connector, and touch the 2 pins with a screw driver or something. If it starts your power button is bad, if not at least you ruled that out.

You can test your PSU by "hot-wiring" it and see if it will run not connected to the mobo. It is not the safest thing in the world, but far from dangerous. Here is a nice guide. If you can power fans and stuff without it being hooked up to the mobo then it's almost definitely not you PSU. If the PSU won't start up when hot-wired, you will probably need a new PSU.

I had that happen. I never did figure out what the problem was.

Sometimes it would fix it's self after i went in there and started unplugging stuff trying to find what the problem was.

Either way i tried 3 power supplies (from working computers), 3 new motherboards and 1 new case.

It would always work after I replaced something, but only for a few months (or weeks in some cases).

I just gave up on it. It's cursed or something.

I did find and artical about it (I lost it though). It just said check the power supply (try one from a working computer), try and start it with out the power switch (touch a paper clip to the to pins the power switch would normaly go over) and if it's not those it's the motherboard and it needs to be replaced.