Bad service by Unitrans Motors Polokwane, operating as Toyota

Good day, I took my car to Polokwane Toyota operating as Unitrans Motors for hazards lights repair. When I went to fetch the car latter during the day, the car did not want to select gears, now the Unitrans people wants me to pay so that they can check what is happening on the gear box. I have never had any gearbox problem with my car, it was only yesterday that I realized my car has stiff gears and gear 4 and 5could not allow me to select it. Please help regarding this matter, my lawyer advised me that I write the memorundum to them, which I did this morning. Is there a way to make this people pay for making my car not to be in a drivable condition?