Your odometer works off of your transmission. The unit that measures the transmission's rotation could have broken. If you want, fix it in a couple months/before you sell it and still claim the odometer was fixed immediately. That way you get more value for your car!
If your speedometer is cable driven from the tranny and the speedo works there is a problem in the cluster itself,,, If the speedo is electronic and the speedo works, check the plug on the tranny or the connection behind the dash cluster