Required support from apple team

hello

on 29 Sep. midnight my iPhone 11 was suddenly shutdown. so i go to apple care and told about my phone. so they collect my phone and after 7 days they told me to my phone has unauthorized modification. but in realty i never do that. i never brig my phone any other shop for any work. and

they did not describe about modification. and don't help me after spending 51500 thousand on this device they did not support me. please support