I hadn't sat at my Pi for a couple of days, so I thought I would do an update.
So I did a reboot first. This took longer than usual, and rather than boot to desktop as it usually did, it presented a login box, with a drop down set to "Pi". I put in the password, the screen goes black, the login dialogue reappears. If I put in a wrong password, I get a red "Incorrect password, please try again", which suggests that I am getting the password right, but the login process is simply not working.
I can only assume that the system has updated itself, which i didn't think it could do, and maybe it hasn't completed properly.