Sometimes ,mostly because you did something , your DB is stuck in a single user mode and you cannot connect to it anymore.
Below steps can help if you don’t know which user is connected to the DB.
SELECT request_session_id FROM sys.dm_tran_locks
WHERE resource_database_id = DB_ID(‘YourDatabase’)
It will return the process connected to the DB with PID
Then kill all process that use your database with following query:
Then run following query to change to multi user mode.
ALTER DATABASE YourDatabase SET MULTI_USER