Unable to Login / Return to Login Screen

My computer was running fine, except for the fact that it was whinging about low disk space because I temporarily copied a folder from an external disk to the internal disk (apparently said folder was a little larger than I was expecting - about 1TB!), then rebooted my computer for an unrelated reason...

Now, when I try to login, I briefly see a black screen, and then am returned to the login screen.

Any ideas?