I don't know what happened. I was trying to back up my system - ironically - and now the SSD won't boot.
It isn't corrupt, it just won't boot.
I can boot from a USB stick and can mount the drive. The directories are there, so it isn't the data is gone.
I now don't have a working machine. I am using it booting via the USB. So no history, no book marks, etc. So it is difficult to go back and show exactly what happened/I did.
What I see is/was indicative of a grub fault. (From results on searches and the replies implying that this would work.) I have read and tried fixing the GRUB. Alas that doesn't seem to be the problem.
On boot, I see the spinning circle for 20.10. The screen blanks, then the spinning circle returns. Then the screen goes black and I have/see a cursor top left of the screen.
Pressing Ctl Alt F3 I can get up a terminal. But then I am stuck.
(Something else I've tried)
I know there is an option to "repair" a messed up install. I start the install and there is an option to repair the install mentioned.
Alas when I get to that page, it is ghosted out. Yet, the third option says it will install Ubuntu 20.10 beside Ubuntu 20.10. I was reluctant to try this, but did start it. But a couple of screens later it is telling me that partition resizing is about to happen. The SSD is a 500GB. I know it has a small partition and a main one. So I am not understanding what it happening and so haven't done that.