Two possibilities.
#1 the cable is rusted up.- Replace system.
#2 The cable end is rusted in the motor cylinder.- It is possible to remove the cable from the motor and lube the motor steering cylinder. This requires that there be room at the transom to pull the cable out. It may mean that the motor may have to be removed or slid sideways to remove the cable. It is sometimes necessary to pound the cable out. First remove the link arm on the port side of the motor, then loosen the cable nut on the starboard side of the motor. Try turning the steering wheel. If it turns and the cable moves in and out to the limits of the motor, the cable is probabily good. If the cable can not be moved it is bad. If you can feel the steering wheel skip, the rack and pinion is damaged and the whole system must be replaced. If the cable moves smoothly, pull on the cable from the nut side of the motor. If it does not move, it must be force out of the motor steering cylinder. The way I do it is to lube with penatrating oil or ATF. Then place a block of wood ( hard wood if you have any) and hammer on the wood. If that does not work, heat the steering tube and repeat previos steps. I keep a fire extinguisher handy and remove the fuel line and/or tank if possible. You can and should lube both sides of the cable. Do not hammer directly on the cable. It will mushroom and never come out. When you get it to move,use a 1/2 inch rod to push/drive out the cable. You will need to wire brush/ sand the inside of the tube and regrease the cable and tube before reassembly. If you have to replace the cable,it is cheaper to buy the whole assembly than it is to buy the cable. You will still need to sand the tube and grease it. You should regrease every 2 years.
Dan, do you have a picture of the transom area of the boat? It might help with the instructions.
Chris