Random shutdown now won't POST

[BartlebyBadger]

Hi,

I built my PC four weeks ago - first one I've built by myself. It ran smoothly up until last night when it randomly shut down. Now it refuses to even POST. All I was doing was checking my email when it shut down so not under any real load.

It has a standby light on but when trying to power up all that happens is the lights come on and the fans spin for a fraction of a second then it shuts down again. further attempts to power it up do nothing. Disconnecting then reconnecting the PSU to the mains supply gives the momentary burst of power again.

Here's my build:
OS: Win 8.1
MOBO: Gigabyte GA 970A DS3P
CPU: AMD FX6300 3500MHz
PSU: Corsair VS550
Memory: 8GB (2x4GB 1600 MHz CL9 DDR3 HyperX Beast)
GPU: Sapphire AMD R7 370 2GB
HDD: Seagate 1TB 7200RPM
CPU Cooler: Cooler Master Hyper 212
Case: Sharkoon VG4W

All parts were brand new for the build.

Reading through various sources leads me to think the PSU might be causing the problem - but without having a spare PSU to test I'm at a loss.

Can anyone give me some advice please? Please keep things simple - I'm new to this self building/maintaining PCs thing!

Thank you.

 

[BartlebyBadger]

A bit of extra info:
The standby light is on an internal card reader which draws its power from the motherboard - this suggests to me that there is power getting through somehow.