For three hours.
At that point, I consulted G who declared that it must have crashed and advised forcing it to shut down and then restarting it. I gritted my teeth (inwardly cursing Microsoft), but everything restarted just fine and after a bit of "installing updates... 30% ... 50%... 77%... it reached 100% and I was actually able to use my computer again. Whew!
However, I spent the morning ironing and reading as well as fretting, so the time wasn't completely wasted.