I had a similar problem with my curve. My phone is through Rogers, and the simple solution had to do with email passwords.
If your email account is NOT a blackberry email account you MUST re enter your passwords every 90 days or the email is permentantly blacked out. Go into "email settings" on your phone, scroll down to existing users, if you remember the name and password you used when you set up your account type it in. If not your Service Provider should have noticed that was the problem and reset your password - call them back and get them to help you with that.
Once that step is finished, follow the directions, enter in all the email addresses and passwords and you should be fine for another 90 days. Just remember the re-enter those passwords every few months!
I hope this helps!!