that's bizarre.

do you have nav?

see if you can get average speeds on either the nav display OR the OBC display.

if you see a speed there, then you either have:

1) lose connection to your heads up display (dashboard)

2) bad Heads up display.

There are usually two or three cables going to the speedo display/HUD unit.

its easy to check if they are plugged in well. remove the two screws at the top of the speedo assembly underneath the hump, and gently wiggle (its tricky) the unit to you and pull it out over the steering column. check to make sure the wires are all secured (the plugs latch down and lock).

if you don't see speed info in the OBC, you quite possibly have a failed speed sensor in the tranny.

good luck