Are you able to Send Emails but not Receive?

Either your domain is out of disk space, or your e-mail address itself is out. You will need to increase the amount of disk space to receive mail again.

Does it say "please enter your username and password for the following server" and you are sure you're using the correct information?

Most of our servers have a 60 checks per hour limit. If you hit this limit in any hour you'll be locked out for the rest of the hour. you will need to click > tools > options > general > and disable where it says "check for new messages every xx minutes" If you do not want to disable it just set it to check less often. keep in mind the more computers you have auto checking or checking the mail the quicker you will hit this limit.