The first place I would look would be where the sonar cable connects to the back of the unit. Make sure it isn't loose. Next check the transducer and make sure it isn't kicked up. No luck yet,
trace the cable from the transducer to the unit looking for a cut. Last option is unplugging the cable from the back of the unit and examining the pins to make sure they aren't bent. Reconnect
cable and try again.
Mike