So I power on (me cup of coffee, computer big button) and log in and up comes the background pic but nothing else. Uh, oh... CTRL-ALT-DELETE, black screen. Eek. Manually switch off, reboot, everything's fine.
It turned out to be a video card two screen problem, it looks like the latest update of Catalyst, the AMD control program, has deviously changed something that isn't happy with my switching off with two screens and restarting with just one.