SQL Server 2008 Mirroring How would one start the secondary mirror instance if the principal SQL server instance went off line?
I'm assuming you don't have automatic failover and no witness, otherwise the mirror would have been promoted automatically.

In this case you need to switch the roles manually, with forced service:

ALTER DATABASE database_name SET PARTNER FORCE_SERVICE_DATA_LOSS


See Role Switching During a Database Mirroring Session and ALTER DATABASE Database Mirroring for more detail.