>>I've run into this when an app - VFP or .NET- is not running, but the VFP or .NET IDE that ran the app is still open. When I close the IDE the issue goes away.
>
>In SSMS, sometimes, an open query window would generate the same situation. IAC, just to sure, whenever I need to restore, I close everything. Then, sometimes, I even have to restart SSMS. When this is not enough, I stop IIS, which might still has some live links to previous database queries. At the very end, a restart of the service. However, recently, not of them was successfull. Yesterday, the switch to single user was the only way to restore.

You can check which connections are still opened to DB

https://social.technet.microsoft.com/wiki/contents/articles/22702.sql-query-to-check-number-of-connections-on-database.aspx

BTW, I often run into a situation when I leave the query window opened and next morning I'm unable to re-connect (all options are greyed out) :(