System not powering on after Windows update.

[Shinnieshan]

Hey guys, this is going to be a bit long winded. It's an issue that's happened before and I managed to fix, thrice, don't know how I fixed it but it worked. For a while, but it keeps coming back so and I'm at my wits end here. 
So just before Christmas I upgraded my Girlfriends PC (Corsair Carbide SPEC-OMEGA, MSI B450M Mortar Max, Ryzen 5 2600, 8Gb corsair DDR4.)
That all went fine, worked like a dream. Sometime in late Febraury Windows performed an update, the system restarted, and didn't come back on. Had a look and the CPU debug light was lit up. Tried all the usual problem solving of removing parts and what not, no avail. Fixed it by clearing CMOS and updating the BIOS. A few days later same issue, Windows updates, PC restarts, but doesn't boot to post. Clearing CMOS and flashing the BIOS worked again. Third time, I just switched off PSU and drained it, left a while and it started back up fine. Turned off Windows updates and she basically left PC on all the time out of fear. 
A couple of weeks ago I went to see her, and to switch out her old PSU for a new Corsair RM750, kinda just hoping that was the issue somehow. 
So I did all that, powered up the system and nothing. Went to bed frustrated and angry and when I got up the next day tried to power on the system and it worked. Restarted it, worked. Updated WIndows, worked. Everything was fine. 
Woke up this morning to a text that Windows had updated and the PC is back to it's CPU debug light issue again. 
And I have no idea what to do next, is it the motherboard, the cpu, the whole damn thing? I don't know.
Anyone have any thoughts on this?