This isn't the huge disaster it sounds: dryden.co.uk went over to Gmail a couple of years ago, so I can still read my emails on the webterface. But for convenience and for having a local record, I do prefer an offline reader.
Googling has not produced a solution, but has produced someone else flagging up a similar problem a few weeks ago using the same client as me (Mac Mail 2.1.3).
I haven't changed any settings, but I wouldn't put it past Mac Mail to have altered something behind my back; it would not be the first time I've been utterly baffled, only to find a box ticked somewhere that had not been ticked by me.
My hope is that it will pop back as mysteriously as it went wrong. If Gmail has suddenly stopped supporting my mail client, however, I might just end up buying a new computer.