This can happen sometimes if you try to take a DB offline or perform certain other operations and they fail.

• Sometimes the lock can be cleared if you close the SSMS instance that attempted the operation, and then reopen it.
• It can also occur if you try to take the DB offline while a long query is running. Follow the below steps to find long-running queries and the kill if there are any.

                     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