Xiaomi India — xiaomi note 4 not starting issues

My xiaomi note 4 which was brought in the month of february went blank a couple of weeks ago. It just would not charge and does not start. While it is still under warranty i went to the thane service center of xiaomi and they indicated that there was damage to the motherboard and i would have to pay 6500 to get it repaired while still under warranty. They indicated something had entered the inside of the phone hence warranty will not cover it. The mi phone is something that we cannot open and i indicated that it was never wet or any external damage had occured but they insisted it was so. Very disappointing that i got to use the phone for just 4 months and now its gone kaput. On reading online reviews found it to be a common problem with note 4 phones. I think the service center guys are fully aware of this issue and want to cover it up. Not happy with the mi experience at all.