manual shifter falling through floor

I was driving to work one day and my shifter fell completly through my floor and now sits on my exhaust. Why and how can I fix this?

Welcome to Automotive Forums. The ball on the end of the shifter (assuming you have a manual tranny) has fallen through the lower ball seat and extension bar (fixed support the shifter goes down through). You will need the upper and lower ball seats and a new extension bar. Here's the steps fo repair it.
1. Remove the shift control rod and nut and bolt from underneath the car. It is on the very bottom of the shifter.
2. From inside the car unscrew the shifter knob.
3. Remove the shift console.
4. Remove the shifter boot.
5. Pry the shifter spring out with a screwdriver.
6. Lift the shift lever and its ball seats out of the shift housing.
7. It's necessary to remove the linkage as well as the lever. Remove the nuts
and washers that hold the shift housing on the end of the extension bar.
Remove and replace the extension bar by disconnecting the other end of
it the tranny.
8. Detatch the shift control rod from the tranny and replace it if it is worn.
.

It would be good if you could get used parts from the boneyard. It would be much more economical than going to the Ford dealer.