Completely Damaged iPhone

On Saturday I experienced some lost signal however, I decided to go to the iStore Canal Walk.It was told there's no real issue they can see with the phone but the assistant will do a update which will cost R200, I said fined lets do it I went off to shopping and on my return the guy advised there's nothing he can do further as he messed up my motherboard there was no manager available at the time so I left, I tried calling however it is impossible to get hold of anyone work at Canal Walk as only US agents answer, I would like this matter to be resolved as my Phone is now useless and I was able to use it before seeing the iStore, I had signal lost and after leaving the store my says iTunes basically a dummy I cant use it at all and all I was told was I dont ****en have to pay the phone is now dead... what the ****??? I signed no documents