Occasional locking of the gear

On the 3rd May 2021, I took my car for tie end rods, heater valve replacement and boot shock replacement. I was informed that the left rack end needs to be fixed. I accepted the quote. Upon collection of the car, my car was not test driven. After I made a payment. I drove for about 8km then my gear locked, I never experience this problem until I took my car to car service city. Today I took the car back to them so they can check what could have happened. They claimed its the gearbox shaft that is a problem. When I asked why this problem started after I brought the car to them, they cannot answer. I phoned their Head office, they obviously did not account and claim it is not them. I am never taking my car there and would not recommend it to anyone