A thump on top of the dash while moving usually fixes this...
Or if you want to keep the miles low - make a note of how many tanks of fuel you put in (approx 220-250 miles per tank), so you know when it's due a service !
I "accidentally" left mine for a couple of hundred lol Tap the speedo and press the reset button at the same time as giving it some death. And you thought mobile phones were dangerous!!
Apparently, this is quite common, when the speedo is going past the 10k/20k etc.. marks. There was a topic on this about 6-8 weeks ago. Not sure how you cure it.
Try reversing a little way. That sometimes gets it working again. Mine stopped on the way down to Le Mans....reversed for about 15 yards on the campsite...voila, working again!!