When restoring a database, you usually start with restoring a full backup, then any differentials, and finally the transaction logs. You set the database restore mode to WITH NORECOVERY, which allows you to perform these additional restores.
But what if you find out that there's nothing more to restore, leaving you with a database stuck in Restoring mode? Then you only need to run a single T-SQL statement:
RESTORE DATABASE [db_name] WITH RECOVERY
Replace db_name with the name of your database.