why does the speedometer stop working
Check the fuses to make sure the fuse hasn't blown. If that's not the case then either the speed sensor is broken of the wiring in the back of the dash is loose.
It could also be a bad stepper-motor on the guage cluster.
Looking for a Used Suburban in your area?
CarGurus has 22,324 nationwide Suburban listings starting at $2,495.
Search Chevrolet Suburban Questions
Chevrolet Suburban Experts
Related Models For Sale
Used Cars For Sale