Could be in the cable itself, or in the 'tube' on the motor.
You might try moving the motor back and forth manually and see if you can get it to turn.
If it still won't move, then you could try disconnecting the steering cable at the motor.
You would then know if it is the cable or at the motor.
I had the same problem last year, and the problem was in the tube at the motor.
Mechanic dealt with it so don't know exactly what he did to free up the 'piston'.
He did recommend that I stop using whatever grease was handy (usually axle grease) on the piston and use a proper marine water repellent grease.
Cleaned as much of the axle grease out as I could and switched, no problem this year,