This has happened a couple of times for me as well, easily solved as described below, pasted from another forum:
You need to reset the speedometer. Here's how to do it: First, remove the negative battery lead. With that lead removed turn the ign switch to the run position. You will then touch and hold the battery cable back to the negative terminal of the bat. Watch the speedo, this should reset the speedo. Now turn off the ign switch and reconnect the battery.
I did this with my Kymco and it resolved the problem easily. Sorry you had to take such invasive measures, but if it happens again try this solution. It worked for me.