I was billed for servive I never requested

My car radio ( BMW X6) was damaged last year so I took it to Dada motors in mafikenf for repairs, I was told that it needed to be replaced. It was then replaced and it was working well. 8 months later it broke down again so i naturally took it back to them to check if the waranty was still active. I was told to leave the car at the workshop. Two days later I was hit with a R 2000 invoice . When I asked what was it for, I was told I wastold that it was for the work done on the car. My problem is that Idid not ask them to do anything to the car, I just wanted to find out if the warranty was still active. Now my car is locked up in their workshop and Ihave no car to drive to work. This is day light *******. Idid not ask them to do anything to the car. WHY SHOULD I PAY FOR SOMETHING IDID NOT AGREE TO?