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Hi and thank you for your question.
My first reaction is that the bendix for the starter is corroded and stuck retracted. That prevents the starter from spinning the crankshaft over.
One thing I would check before that is whether the crankshaft turns or not. To check that, remove all the spark plugs, remove the cover over the coupler at the rear of the engine and grab the coupler with your hand and see if the crank will turn.
If it does, it is the bendix. Yamaha calls it the Idle Gear Comp.
If you give me the exact model I can send you a link for a diagram of it.