Hi Jennifer,
If you have the access or credential of database you can do a manual entry in mdl_user table and log in to site. You will get you Moodle database credentials under Moodle root dir in config.php Also you have to check mdl_config table, it has a row as siteadmin who's value is the id form the mdl_user table. It specifies which user is a site admin. So if the user already exist in user table, just change the siteadmin value and you should be able to log in.
Ambrish Tiwari