If you are using SQLServer probably you had to restore a database from a backup and got stuck with an orphaned user that was no more associated with a login.
Microsoft provides a stored procedure to remove this orphaned user:
exec sp_revokedbaccess 'Orphaned_User_Name'
You could get an error because the user still owns a schema. To find out which schema is bound with the user you can use this query
SELECT name FROM sys.schemas WHERE principal_id = USER_ID('Orphaned_User_Name')
And then drop the orphaned user from the schema
ALTER AUTHORIZATION ON SCHEMA::SchemaName TO dbo GO DROP USER Orphaned_User_Name