RAID1: What happens when one half of a mirror fails?

Hello, All:

I've been a bit surprised by the complete failure of our system when one half 
of a mirror failed so I suppose that I should verify that I understand 
RAID1's failover methodology: Is the purpose of the mirror only to preserve 
the data or is it to allow the system to continue operating until a 
replacement can be installed? (i.e., If one half of a mirror fails, shouldn't 
the system continue operating normally?)

-- 
Eric P.,
Truckee, CA