If an account has disappeared from 'Main >> Account Information >> List Accounts' in WHM then one of the following steps should help correct the issue:

del -f /etc/trueuserowners
/scripts/updateuserdomains

You may also want tocheck the entries in the following file:

/etc/trueuserdomains

If the entry for the domain is missing then add it and the username for the domain will be listed under the WHM.