This can happen sometimes if you try to take a DB offline or perform certain other operations and they fail.
Ø Select DB_ID('DB_Name')
Ø Select status, session_id, command from sys.dm_exec_requests Where database_id =<dbid>
After running above query, If you find the status column with value - "Suspended" then you have to
kill that session id. You can run below query to kill that spid.
Ø KILL <spid>
If neither of the above works, close all SSMS instances then restart SQL Server Service through the SQL Server Configuration Manager. Usually that will cure it, although the DB may be in recovery mode at first