Speedometer not working 1999 model 206?

My car kept cutting out when i slowed down eg at lights or just as i changed from gear 1. It started again when i turned the key but the next day i noticed the speedometer not working and the digital mileage counter had gone down to a few hundred miles instead of around 70,000. The cutting out only occured for 2 days and has not happened since but the speedometer is not working at all. I tried to get a diagnostic form a garage and the RAC, but both computers could not communicate with my car's computer. Any ideas what the problem may be? Sensor? speedo, communication? connection? and how much it may cost to fix.