unfair billing

My car was towed in to Hyundai because the ignition was faulty and therefore the keys couldnt start the car. The car is currently under warranty so to my knowledge as informed by one of the employees he mentioned that I wouldnt have to pay because it is still under warranty and one cannot prove that it was not a mechanical issue. However I got a call from them this week and they were quoting me R7200 and mentioned that the issue is not a mechanical issue, of which to me doesnt make sense how can I have faulted the ignition?.please help me my registration number is CRW 932 L Car model Blue Hyundai i20 2012 model my name:Lebogang Matlala