Better to say it at start: it all finished with fresh Windows 10 install, but I would like to hear opinion or two, since this adventure was quite a surprise to me.

Couple of days ago, friend called me and stated, that his PC won't boot into Windows (was 8.1 at the time, I know this is 10 forums, but it was solved with 10 boot USB drive).
I spent one hour with his PC (instead of having lunch) and to my surprise, no solution worked:
- all troubleshooting options (reapir boot, Refresh, Windows Reset, System restore...) ended with fail and option to restart.
- managed to get into command prompt (both safe mode and with Win USB) but diskpart didn't recognize any partitions at all (I was on drive C: checking files
- bcdedit found no Windows installation - tried every option possible, but couldn't transfer or set any boot files ..

Finally started with Macrium (my Win 10 version) USB drive and made image of disk in question on portable drive, verified that all the files are really there and readable and did a fresh Windows 10 installation.

That decision was made after my spare time was gone and friend finished installation. I instructed him later to install Macrium, mount image and restore files that he needs.

Checked Google later and was surprised, how many of those problems are out there. Most of them are solvable, but this one resisted all of solutions that I later found. Didn't tried only one - copying registry from registry backup.., but at that stage I was going for 100 % solution, as there was no time. Also asked myself, if I would run a system that behaves like that.

Also asked friend what was he doing... only thing he remembers (PC was running at least three to four days without restart) that he uninstalled some ISO maker and then restart...

So, did I missed anything important? Outcome is happy user (Macrium installed and system backup made...) with all of his files there, but upgrade wasn't planned and some time was spent...