Over a month? Are you sure they didn't cancel your account?
In all seriousness, if you can still access your account through the web interface (with all the other goodies), then you should contact .Mac tech support. Also make sure that you didnt change some setting in your Mail.app. Remove then try to setup your .Mac account again.
Yeah, it happens sometimes with my own email provider too. It's a small bug their end that shows up with this password dialogue box. As far as I know, there's not much we can do about it at our end. After a while it should just go away.