Not completely sure what you mean by "can't turn" but if you mean that the system only responds to the buttons pushed for one direction and not the buttons for the other direction.......
I had a similar problem last year.
First we tested to see if it was a problem with the buttons (control head). You can "test" them by checking if they respond to commands in other modes - for example can they be used to reset parameters in the calibration and options modes or to control the lighting functions? If so, then it is unlikely they are only malfunctioning in the steering mode eg it is not the control head acting up.
You can test the drive moter next. Mine was a simple dc motor - turned one way or the other based on the polarity of the control signal voltage. Rig 12v test leads and (after removing the control cables) try to see if the motor works in both directions by applying current to the motor and then reversing the polarity.
If the motor works, check the voltage output at the control leads. Activate the buttons to initiate a turn and measure the voltage at the motor connections. The SeaTalk course computer modifies the voltage to control the speed of the motor. (I didn't even know I had a ST Course Computer! It was hidden completely in the port stern storage compartment way up out of sight. It took 2 hours of tearing apart everything on the boat to find it!)
In my situation, the buttons and the motor tested out fine but the voltage was too low to activate the drive motor - even worse in one direction than the other (2.5 volts). The technician performed a hard reset on the SeaTalk course computer and VOILA -- everything worked again. Of course all was restored to factory presets and had to be reprogramed and calibrated. Not sure what caused the original problem other than it is a computer -- what more is there to say? Good reason not to rely completely on electronics I guess.