HP zbook 15 G1 Boot failure / automatic repair loop

[TTpie]

Hi!

I have a HP zbook 15 g1 workstation which fails to boot / is stuck on automatic repair loop. I cannot for the life of me locate the problem. The original warranty is long gone as is the additional warranty the seller gave it. I’d like and try to repair it if at all possible though.

When I turn it on Windows tries to boot normally, fails, boots into “Windows preparing automatic repair”, it fails and tries again, fails, tries, fails etc. Sometimes after the first “preparing automatic repair” failure the screen will stay black and the keyboard light will blink on and off for a while, before the whole thing begins again.. Hard reboot doesn’t help. I cannot access the Advanced Startup options or Command Prompt to try and troubleshoot this because the boot fails before those would be accessible (at the preparing automatic repair stage).

I can access the BIOS but pretty much nothing else. BIOS diagnostics say that both the memory and the hard drive are working properly.

I have tried booting it from an ISO file on an USB stick, but the same thing happens. It will not boot.

I bought it used, and the original win8 pro (64) had been updated to win 10. Is it a hardware problem? Or software? How best to troubleshoot it?

EDIT: Forgot to add that before this I had been running some pretty big photogrammetry projects etc. on the computer which  might have led to hardware failures. There's 16 GB RAM, on the slots under the keyboard i presume as the additional two slots at the back are empty.The hard drive is Samsung SSD 860 EVO (500gb). Memory problem? Graphics card problem? How do I find it out?

Any help would be greatly appreciated!

TT