The speedo is powered through a mechanical linkage from the tranmission.  It it typical for the speedo cable to fail.  Check to see if the cable is not loose on both ends.  The instrument cluster end is very hard to detach and reattach, and messing with this end will reward you with an excruciating job of getting the end to connect back into the back of the instrument cluster.  The other end snakes through the firewall, down under the car, and to the back end of the transmission.  Make sure this connection is not loose.

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