http://cleanflametrap.com/speedoDiag.html
http://forums.turbobricks.com/showthread.php?t=242180
http://www.davebarton.com/240-odometer-repair.html
Not sure if your car is manual or electronic speedo, but in order of probability, it's either the cogs in the speedo, a broken wiring connection, or faulty electronic sender in the differential. Far less likely is the power supplying the pod, but it's unlikely especially if other gauges are working.
Where to start depends on what you find easiest, I guess. ;-)
You can check wire connections (continuity with a multimeter), or the plastic cogs inside the speedo first - they're sold on ebay, or there are free online plans to 3D-print them. And you can check for a pulse from the diff (if it's electronic speedo) with a multimeter that has a frequency setting.
I think I saw a youtube video a few weeks ago showing how to test the sender by raising the rear of the car on stands and spinning the wheel by hand. But I've basically just described it without needing the video. Pull the wiring plug at the diff, stick the meter probes in the plug, turn the multimeter to frequency, spin the wheel and the numbers on the screen should change. I think it's left wheel you spin - anyone confirm?
If you don't have a multimeter with a frequency setting, Aliexpress has cheap ones if you can wait about 3 weeks. In the meantime there are free speedo phone apps while you troubleshoot.