First signs something was wrong : a file wouldn't save and a browser bookmark was not recorded.
du -h
Said the partition was 100% full. I didn't think that was true, but I deleted 4 full-feature movies (with the key, so they don't just go into the trash). I emptied the trash anyway. The above command still said the partition (I have only one working partition, with Lubuntu) was 100% full.
I thought the computer was simply playing up and I rebooted. Or tried to.
When I try to boot, I see the splash screen, then some text (this is normal) and the mouse pointer appears (and moves when I touch the scratchpad). Nothing more.
Pressing F12 gives me a boot menu with Ubuntu, Windows (was overwritten but still has some stuff in /dev/dsa1) or UEFI. No advanced options or recovery mode. Any of these 3 options end up as described above.
Pressing escape repeatedly while booting gives me a prompt :
grub>
Following instructions here: howtogeek.com/887757/how-to-use-grub-rescue-to-fix-linux I typed
grub> set root=(hd0,2)
grub> linux /boot/vmlinuz-6.5.0-28-generic root=/dev/sda2
grub> initrd /boot/initrd.img-6.5.0.28-generic
grub> boot
This said there was no hda2 and gave me another type of command prompt, this one was called
initramfs>
Typing
initramfs> exit
goes back and changes the keyboard layout back to azerty. So it has found hda2, otherwise how could it know I have an azerty keyboard? Before I had to type in qwerty on the azerty keyboard.
Searching online returned many post talking about new installations. This is not a new installation and I don't want to loose the stuff I have on that computer.
I also found lots of advice using normal command-line instructions. Please don't suggest any of this as I cannot access any bash terminal. I can only access grub> and initramfs> command prompts and the language is very different.
The closest is : Boot drops to a (initramfs) prompts/busybox but it doesn't answer my problem.