i. ALTER DATABASE <DBNAME> SET ONLINE
ii. EXEC SP_RESET STATUS
iii. Before doing this make sure that we inform the DB Users that we are going to work on it to make necessary troubleshooting steps.
iv. Reason can be found in SQL Server Logs why/how/who has taken the database offline.
User Unable to connect
Slow response from the DB when user connected
DB is in Restoring state