To fix the user and login mapping you need to open a query window in the SQL Server Management Studio. Enter the following two lines and replace myDB with the database name and myUser with the correct user name:
USE myDB EXEC sp_change_users_login 'Auto_Fix', 'myUser'
If run successfully you should get an output like this one:
The row for user '****' will be fixed by updating its login link to a login already in existence. The number of orphaned users fixed by updating users was 1. The number of orphaned users fixed by adding new logins and then updating users was 0.**
Your user should now be mapped correctly.
New way to Resolve/Fix an Orphaned User:
In the master database, use the CREATE LOGIN statement with the SID option to recreate a missing login, providing the SID of the database user.
CREATE LOGIN <login_name> WITH PASSWORD = '<use_a_strong_password_here>', SID = <SID>;
To map an orphaned user to a login which already exists in master, execute the ALTER USER statement in the user database, specifying the login name.
ALTER USER <user_name> WITH Login = <login_name>;
When you recreate a missing login, the user can access the database using the password provided. Then the user can alter the password of the login account by using the ALTER LOGIN statement.
ALTER LOGIN <login_name> WITH PASSWORD = '<enterStrongPasswordHere>';