Computer randomly shut itself off


TS Rookie
Hi, I'm not sure if this is the right section to post this but I had this problem occur today, and well my computer as you can guess by the title randomly shut itself off..

i Don't know why it did but im guessing it was the power supply/Mobo nothing was wrong with the mobo though, i took apart my computer and looked but there was nothing wrong with the cpu/mobo so the only thing im left with is either the heat or psu is burnt out.

Pc Specs:

Intel Core 3.3ghz
gtx 550ti Gpu
Biostar TZ68+ mobo
and my psu if you were wondering, is COOLMAX 650w..

so if someone can help me that would be great, by the way nothing turns on at all when i hit the start/reset button ive also tried shorting the pins on the motherboard for the power but nothing not fans or any beeping/noises.