Never modify or jailbrake my IPhone 5 since I bought it from the authorized dealer, but the service center said it was!!!

Last week, I sent my IPhone to the service center after I dropped it in the water, and could not turn it on.

I honestly never modify or jailbrake my IPhone 5 after I bought it from the cellphone carrier shop (True Corporation. Thailand who is an authorized IPhone dealer).

But after a week after sending the phone in at the cellphone carrier shop, the authorized service center for IPhone in Thailand said it was modified, and not eligible for warranty coverage. (If they are convinced that the phone was never modified, I will still need to pay about $300 for a replacement of the same model)

The apple's IPhone authorized service center did not give any reason why they say my phone was modified, which surprise and upset me.

What should I do? Any recommendation or a place I can contact?

Pat Nanta