Chris,

Sounds as if the central supports have failed. Does your house have
central supports? If not, why not? If yes, why have they failed?
Most probably someone built the house without central support and the
house has sagged as you describe. If that's the case you'll need to add
central supports. These can be the jack posts that everyone has already told
you about. The jacks need to be supported by some sort of foundation.
A civil engineer can tell you how many jack posts, where to place them
and what kind of foundation. Or tell you why the house is sagging. You'll
find engineers in the phone book.

Dave M.