I'd check the transducer to see of the paddle wheel is moving first, and then check the transducer connections to the display head. in the absence of transducer input the unit is smart enough to figure out to go to repeater mode automatically. so it sounds like either the transducer is bad (check for ac voltage while someone spins the paddle wheel, also check for continuity between the two wires and for not connection to ground in either wire) or the transducer connections to the display head are faulty (if memory serves there are only two and while some will say the you can connect them backward I'm not convinced of that since the signal is just pulses of AC) or the connectors are corroded or wires broken. I believe the unit can tell if there is a transducer by the presence of a circuit (continuity between the two wires) when it starts up and there does not need to be actual data coming over the wires.
I'd look here first as this is normal when transducer data is not available.