In the past, most mechanics ended up having to replace the speedometer head when a vehicle came in with a broken speedometer. Nowadays, there's a possible reset procedure that can be used, and it can be done by most car owners right at home.
A common problem for this malfunction is when the car owner recently replaced the battery, or perhaps jump-stared their vehicle, which in both cases could have caused an electrical spike that made the speedometer go berserk.
See the simple reset solution in the video below, demonstrated on a 2002 Chrysler Sebring. Other make and models may have a similar solution.