Next I typed 'journalctl -xb' to view system logs : a dozen of pages
which I will shoot later, one thing I have noticed in it all, printed in
red characters :
EXT4-fs (sda2): unable to read superblock
this repeated three times !
This looks a bit worrisome, but it may not actually be; it depends on what device is at sda2 (it might be a second (unformatted, unused) hard drive, or a USB thumb drive, or a CD/DVD drive, or ...).
No it can't be; I wasn't thinking straight. This is a second partition, not a second drive.
This is almost certainly your problem. The next question is if it's a hardware, formatting, or mounting problem. I'd run "cfdisk" and see what that shows me.