No it didn't get hit by something. There was some dust on the frame I was cleaning that and there were some sticky grains of dust, I tried to get them off by my nail & the paint started peeling off.
I know everything about the Phone but I bought it 2nd hand that's why I was doubtful about the back being replaced and it has no water resistance. But I'm satisfied because I put the Phone in water and it is alright. Thank you everyone for the help!
But mine is not from US. I have seen many pictures and watched videos, some of them have only IMEI on the back and others have both IMEI and serial numbers.
I have rooted Samsung galaxy s6.Last night i searched google for unroot and remove super su app.Then from root explorer i deleted Supersu.apk file and SU file and restarted the device then it stucks on samsung logo with blue led blinking.