When restoring database on different MS SQL server, DB connections will be rejected unless 'orphaned' user is fixed. This applies even if the same username + password exists on target MS SQL Server.
You need to have appropriate permissions (DB admin) to be able to fix the 'orphaned' user.
EXEC sp_change_users_login 'Report' will show list of orphaned users
EXEC sp_change_users_login 'Auto_Fix', 'username' will fix orphaned user in case when same username and password exists on target server
EXEC sp_change_users_login 'Auto_Fix', 'username', 'login', 'password' will fix orphaned user and create new login on target server.