This post is to help others avoid what I had to go through for the past 3 weeks. I think it's DISGUSTING that HP have ZERO support on this matter, so I hope this saves someone's day.
All credit goes to everyone involved on this thread; https://h30434.www3.hp.com/t5/Desktops-Archive-Read-Only/Possible-bios-recovery-problem/td-p/5208527...
After a BSOD after I played around with my USB Windows to Go (Windows 10), my HP 500-023w failed to even boot up into BIOS. Just a blank screen but with fans and disks spinning.
I replaced the RAM, CMOS battery, and PSU (having trawled through HUNDREDS of forum posts), but still no luck. I managed to get my computer to boot, but it took ages, and it was a hit and miss (sometimes booted, sometimes didn't).
Long story short (3 weeks of trial and error), I flashed my BIOS to the latest version, which helpfully, is NOT on the HP support website (bunch of useless toads), and everything works again. I'm RMA'ing all bought components at a restocking price.
The fact that the BIOS update on the HP site is an old update that fails your BIOS, and the fact that you have to search their FTP to get the latest version, is absolutely disgusting and unacceptable. If you have a 2ADA MOBO that "failed", I would recommend you update to v8.2 before you splash out on any components. Worse still, before you reach out to HP support, who will ask irrelevant questions and generally be useless in not answering any of them.
Never buying an HP again.