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.
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
.newat the end
.baksuffix from the other one
Reboot your machine. It should start applying a Windows Update
Log in, everything should be fixed