incoming icloud mail server is blank...works on iPhone but not on mackbook. Help!

My @me.com inbox has worked great until this morning. I tried to send an email and is says my incoming mail server cannot be blank. The 'example' that is entered into that field is greyed out and I cannot edit it.

My @me.com mail works fine on my iPhone.

I have icloud enabled with mail on my macbook. Help please!