Sounds like the small electrical motor driving the trip have stopped working, I had the same and got another speedo from a junk yard and just swapped that tiny motor over ... once speedo is out it's held in by just two screws and a electrical connector to the PCB in there and it's s straight forward easy swap.
I might have some photos from when I did mine if that will help you.
The "donor" can be an 87-92 120 or 140mph speedo