PC Won't Boot (Potential PSU Problem)

[Guydude]

So I built a PC for my brother and it wouldn't power up, so I thought the PSU wasn't big enough so I tested this and put my 650W PSU in and the PC booted but I need my PSU so I had to take it back. So I went to BestBuy and got a CX750M PSU and plugged it in and it did not work, so after that I tried to see if my PSU would work again and it did not. The 750W one works I tried it in my PC I just don't know what to do.
Specs:
GPU: GTX 550 Ti
CPU: i3-3250
Mobo: ASRock B75M-DGS R2.0 Micro ATX LGA1155
RAM: Corsair Vengeance 16 GB

 

[digitaldoc]

Keep the 750W in your PC if this is working.

Get a new PSU for your bro's PC. You should be able to use a 500W PSU.

 

[Guydude]

Keep the 750W in your PC if this is working.

Get a new PSU for your bro's PC. You should be able to use a 500W PSU.

I don't see the point of getting a new PSU if the one I had in my PC work perfectly before.

 

[digitaldoc]

I think the key word is "Before." It sounded like it was working previously, but not currently working. Try it again, but it sounds like it is not working currently.