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Given that it was intermittent, and that the transducer is now thoroughly un-reachable, I would say that checking the connection is the first thing to do.
Check the cables along their entire length, the cables between the transducer itself where it is mounted, and the Map198c device. See if they show any signs of damage, breakage, perforation, severe bending, etc.
Then, for all the connectors along that cable between transducer and Map198c device, undo one connector at a time - and scrub both sides of that connector with a clean dry brush, for 20 seconds+ each. Then wipe both sides of the connection down with a clean dry rag cloth, until the metal shines. Then re-connect firmly, and go on to the next, until all the connections are done.
Then turn the system On again, and see how it does.
If the sonar transducer is still unresponsive at this point, tell me what other devices the Map198c is connected to, and consider the transducer tentatively suspect of having gone bad.
(If the above advice gets the system working again, please rate my answer. Thanks.)