Also, the cable may have not been tight in the back of the speedometer and worked itself loose.

It is very easy and quick to check it. Just push out the instrument cluster and see that it is tight. If it is, then remove the 10MM bolt at the trans, pull out the cable and spin the cable with your fingers clockwise while an assistant observes the speedo. Spinning should result in speedo needle movement. If not, remove the cable from the back of the speedo and put a kitchen match in. Spin the kitchen match clockwise with your fingers and see if the speedo needle moves.

Good luck,