Check and double check and make sure your connectors are not cross threaded and connected tightly and in the correct slots on the back of the unit. If this is not working then the transduce may be bad or the cable it is hooked to has a break in it somewhere. The transducer also must be in water to get a reliable test and the transducer must be a match to the unit. If yours is not a match then Humminbird offers an exchange for the correct one for your unit.You can check the pins where they go into the unit on the quick disconnect mount. Look to see if the female pins haven't pushed up and in, also that the males are not bent or corroaded. I doubt it, but its a quick check.
Remove the head unit from the quick disconnect mount. You will then find all the connectors held in place with two screws. If the screws are loose, the connectors can pop out and down. Also, check the pins when you have it off.
Here is a manual, in case you don't have one: http://www.humminbird.com/PDF/100sx.pdf