I backed up a database:
BACKUP DATABASE MyDatabase
TO DISK = \'MyDatabase.bak\'
WITH INIT --overwrite existing
And then tried to restore it
In my case, it was sufficient to drop the database which was hanging in state "Restoring..." with the SQL command
drop database
in a query window.
Then I right-clicked on Databases and selected Refresh which removed the entry in Management Studio. Afterwards I did a new restore which worked fine (note that bringing it offline did not work, a restart of the SQL service did not work, a server reboot did not work as well).