Menu bar disappeared

I've just had a weird experience with my MacBook Pro (OS Monterey, early 2015 MBP):

Firstly the menu bar disappeared and I couldn't get it to appear. Start / restart a few times

Then the contents of my hard drive couldn't be accessed (greyed out in the Finder). If I clicked on any files, I could not access them. The Apps continued to work after a fashion (Adobe DC wouldn't let me create bookmarks, but oddly would let me highlight text). I could send documents from AirDrop but couldn't save other than in the Downloads folder.

Then the keyboard stopped working. The mouse worked throughout.

I'd assumed that the (7 year old) MBP was now, er, beyond saving.

But, I tried re-booting in recovery mode, ran Disk Utility on both the general and HD drives (nothing found) and then I re-booted in Safe mode.

FIXED