There are tons of reasons why email would stop working. Contact your service provider first, make sure it's nothing on their end. And be considerably more specific, e. g. the program you're using for email (Outlook, Outlook Express, etc.), the computer you're using (Mac or PC), operating system (Windows, Mac, Linux), and whether you have changed anything recently or are experiencing any other issues.
If it's not their issue, be prepared to recite the same thing again to your email provider. These specifics are really needed in order to give the technician a way to help you without just throwing darts at a board.