Power not working Dell XPS 8900

[davelad1999]

So my computer was working fine yesterday, and has been for the last year (My Dell XPS 8900 has a GTX 1060 which i installed myself over a year ago). However, it wouldn't power on today. Dell computers have a BIST (Built In Self Test) buttons on their PSU's, when i hold this button down, the light flashes green which according to the website means working fine, but as soon as i let go the computer loses power. Just pressing the BIST button does nothing, I have to hold it down. The main power button on the top of the case also doesn't work.

Basically, is my PSU broken? Or is it something else. (Edit. NOTE: When I hold down the BIST, the Graphics card fans spin, the hard drive boots up, but the CPU cooler fan and the fan mounted on the back spin for 1 second then stop, perhaps a motherboard issue?)

 

[hang-the-9]

First thing I'd try is another power supply. No way to guess for sure without replacing parts and see what fixes it.