You can't do much - the server has to be fixed. Is it a google.com account? (If it is, you might have something set up wrong on the phone. Delete the account, then add it back [as an existing account] again. Also, use a web browser to go to
GMail and see if your mail shows up there.)
If it's some other email provider, it could be an incorrect setting in the phone, it could be the server having a problem, there's no way to tell without calling the email provider.