When I booted Windows today, I was presented with a message that says that I've been given a temporary profile.
Most of the instructions I found revolved around running
sfc /scannow, but when I tried that I was told that the system was already locked by another process.
I checked when Windows Update last ran, and it looks like there was an update last night, which seems like it might be something to do with it. A bit more googling took me to this page which I'm going to summarise here for myself in future / in case that page disappears.
regedit as administrator
Find the path
Look at the keys in this path. Two of them (they're quite long names) will be named the same, but one will have a
Rename the one without the suffix to have
.new at the end
.bak suffix from the other one
Reboot your machine. It should start applying a Windows Update
Log in, everything should be fixed