MSSQL - Restore DB and Reconnect Orphaned Users

First, make sure that this is the problem. This will lists the orphaned users:
EXEC sp_change_users_login 'Report'

If you already have a login id and password for this user, fix it by doing:
EXEC sp_change_users_login 'Auto_Fix', 'user'

If you want to create a new login id and password for this user, fix it by doing:
EXEC sp_change_users_login 'Auto_Fix', 'user', 'login', 'password'

http://support.microsoft.com/kb/274188