Mine stopped working recently, I think it is just a problem with the 12vdc power to the unit. I am in the process of cleaning up the wiring this winter, I will post whatever info I find.
Is the depth finder display available in the gauge window? If the box is powered up and the device is recognized by the MMDC, there should be a menu selection for the depth finder and a depth displayed, even if it is a bogus number.
If that is the case, the transducer (glued to hull in bilge somewhere) probably came loose or quit working. Try peeling it up and gluing it back down, someone may have glued it down with silicone.