I'd had a long, tiring day yesterday, and wanted nothing more to get home and relax. I woke my laptop at home to check my email quickly, and found that it hadn't connected to the wireless network. When I tried to connect (it listed our network as available), it gave a rather cryptic error. Odd. So I went into diagnostic mode: the iMac was connected to the same network wirelessly with no problems, so it didn't seem to be a problem with the router. I double-checked passwords and settings, and they seemed fine. I connected directly to the router via ethernet, and that let me connect. Finally, I tried my old trick of logging in as a newly-created user, which helps to isolate the problem if it's an error with a preference file in your own user folder. Still no filled Airport signal strength segments, which would have indicated that my laptop had rejoined the wireless party.
At this point, I was gloomily thinking that my wireless card must be broken, and I'd have to get it fixed, and also that I was blowing most of the evening getting frustrated with a technical problem, neither of which were good things. I decided to have one last go. The Network pane of System Preferences has a button at the bottom labelled 'Assist me...', so I thought I'd give it a try. You then get a further dialogue with two choices: 'Diagnostics' or 'Assistant'. I tried Diagnostics first, and got taken through the steps to check the Airport card, including turning the router off and on again (which would have been greatly improved by an audio clip of Roy from IT Crowd saying "Have you tried turning it off and on again?" in an Irish accent). At the end of the process, it shrugged its shoulders and said it couldn't fix the problem and didn't know what the problem was. Sorry. Thanks very much Diagnostics.
As a last resort, I tried 'Assistant'. It politely walked me through the steps to set up a new connection, which I didn't expect to work. But lo and behold! At the end of the process, I was connected again.
I'm not sure what the moral of this rambling story is, but if you find yourself with weird network problems, try Assistant before resorting to Diagnostics (aka Roy), and you might have more of your evening to enjoy. I still don't know what the original problem was, but I suspect that my laptop was sulking because, being UK-based, it didn't have a new iPhone friend to talk to on Friday night.