BMW Montana a great disappointment

I recently took my BMW F30 for service and there was no problem with my lights. In the afternoon when I collected the vehicle, I then realised that my right front light was not working as well as my indicators. I then went back to the workshop to establish what happened to my light and why my vehicle was showing light errors. They then printed a report showing that the control unit was dead, which was not the case prior to them servicing my vehicle. I now suspect that they swapped my control unit with another one. They diagnosed the vehicle before the service and there was never a time where they raised the issue of the control unit being dead. I'm really not happy because according to me, BMW is a reputable company and they refused to take accountability for my vehicle.