Assign database user to Login Name

If you have migrated your database, it is likely that the user will not be functional or cannot login. In this case, create a new login on Server Level and issue the following stored procedure to attached database user to login name.

Sp_change_users_login @Action= 'Update_One', @UserNamePattern= 'UserName',

@LoginName= 'LoginName' 

Reference : Add Database User to Login Name – Microsoft