What version of Outlook express do you have?

it is possible that your account has become corrupted some how...

Also, you might try going to tools accounts and removing your mail account...and then just rebuilding it...but first make sure you have the right settings...those can be gotten from the servers tab of your mail properties....

what you might also want to try is to go to file and identities...try creating a new identity to see if it works in there....