In relation to this, what happens if one of the site's goes down (i.e - the principal site) and automatic failover is not being used (i.e. - there is no witness server).

In this case, how would manual failover be done on the principal server if the entire server is down/disconnected from the mirror server?

If failover is not done manually and automatic failover cannot be done anyway then what happens with mirroring?

Does it continue running?

Also what happens to the mirrored databases? I'm assuming they remain as mirrored and therefore inaccessible by users?

Thanks.