Cannot remove Mirroring / nor restore or delete database:Error 3456 (SQL Server 2012)
First shutting down Windows without stopping SQL Server first didn't cause this.  That won't do anything bad to the database as all services are cleanly shutdown when Windows is told to turn off.

I assume that the principle is offline and can't be brought online?

Try forcing the database to come online.

ALTER DATABASE X SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS


That should bring the database online which should then allow you to remove mirroring and drop the database (or do whatever else you need to do with it).