There are several informative posts on this topic, if you care for more detailed information. Basically, what almost always happens, is that a small blue gear loses it's teeth and doesn't turn the odometer and trip meter anymore. It's extremely common for 90-95 odometers..none of my 89 assemblies have failed though!
(They're made by a different vendor - you can tell if you look closely when installed. If you take them out, it's obvious, since there are different names stamped on them)
To fix: you can buy another gear (around $25 I think) and fix it yourself (a little tedious, but not too bad) or find another speedometer assembly that works. Any 89-95 TBird 0-120mph or 0-145mph speedometer will slide right into the gauge assembly. Of course, then you have an odometer descrepancy....but wouldn't you anyway?