Not sure what you mean by "reset your preferences," but if you are using the Mail program, open Mail. In the Mail menu, got to Preferences and go to the Accounts section. Is your mail account listed on the left? If it is, click it and check all your settings. If it is not listed, click the + sign at the bottom to create a new account with your account settings. You may need to contact your ISP if you don't know the correct settings, but most ISPs have these settings published on their support web site.
Steve