Here is the script you can use to bring the mirroring database out of its “Restoring…” coma:
alter database MyDb set partner off
restore database MyDb with recovery
This is useful when you have high performance mirroring setup, your principal blows up, and there is no witness present. At this point, you will have to use the script, because you will not be able to access the mirroring database. Notice that the automatic failover does not work in this case. Applications and users need to point to the new database to resume their work.
Thanks to the folks who asked me those questions.
While I am on the topic of mirroring, the mirrored database must have the same name as the that of the principal, at least from my experience so far.