If the database is in restoring state run the command below to bring the database online:
RESTORE DATABASE <Database Name> WITH RECOVERY
If the database is in read-only (standby) mode you can bring the database online in two steps
Step 01: Disconnect all the connections to the database
USE master
ALTER DATABASE <Database Name> SET OFFLINE WITH ROLLBACK IMMEDIATE
ALTER DATABASE <Database Name> SET ONLINE
Step 02: Bring the database online
RESTORE DATABASE <Database Name> WITH RECOVERY
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