This happened to my 97 Mustang a week ago. Everything works EXCEPT the odometer and the trip odometer. The dealer says I need to replace the entire cluster, because the gears for the odometer broke. They said the plastic they're made of tends to get brittle and eventually break. The dealer wanted over $500 for the cluster.
I just ordered a cluster for a salvage dealer that Autozone works with, EverDrive, for $118.88 to include shipping to my house. It has a 1 year unlimited mileage warranty and takes about 5 business days to get delivered. The number on my receipt for them is 877-827-1188.
If you're not interested in that, then i have 2 other sites listed below for you.
http://www.simcoltd.com/shop/mustang/ - new aftermarket cluster
http://www.mrwhizard.com/services2.cfm - cluster repair