Ah... Linux. It doesn't fail me too often, but when it does, it's a nuisance putting things right. I've had the same Linux installation on my main desktop PC at home for 1.5 years now, and it's mostly OK.
I just ran Linux update, which included a new kernel version. Then the reboot I did after that failed, telling me one of my partitions had filesystem corruption. I had to run the fsck utility in manual mode, and fix each error in turn. Then after the normal reboot following that, my 2nd monitor wasn't detected by Linux. Fortunately, I'd seen that one before, and I simply rebooted again, and now all is normal again, with my Linux desktop spanning the 2 monitors.
Of course, I didn't break PC Rule #1, which is to do a data backup before anything like a major software update (especially one with a new kernel version in it), so I would have been able to restore my data if all had gone pear-shaped.