Can send from MobileMe, but can't receive. Help?

I've been using Mail since last Oct. and haven't changed a thing. Suddenly, I can only send from my MobileMe account, but I cannot receive through it. This occurs using the mail app and ALSO when trying to retreive mail using the browser-based option at me.com. We keep sending mail to my addy, but nothing comes in when checking mail and nothing bounces back. We're stumped. Outgoing and incoming mail server settings are the same as when it worked previous to this. Please help. Thanks!