There is a circuit breaker on the main battery and it can fail. It will be a 20 Amp breaker on the Red 12 gauge wire within 6″ of the main battery post. Check the terminal block on the rear and figure out if a screw or wire has deteriorated or loosened. That is located on the transom. Determine if you have power to that terminal block, the 12 gauge wires (Black and Red) then run from that terminal block to the Master Power switch area. Pop the switch to figure out if you have power there. It could be corrosion and I may be a simple loose wire or faulty switch. Do not over focus on the Red wire (+) as it could be the Black Negative (-) also. Year & Model?
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