The speedo is about $900 from the dealer, so definately check the speedo drive first!
It's possible that it was reassembled incorrectly on a tire change and you've got a stripped drive plate (the cheap part!) but could be a broken or stripped worm gear.
If it's the worm gear, you're probably going to need a new speedo drive. Check with
www.baxtercycle if that's the case since they might have a used replacement.
Jim