My iPhone 6s pluse reset about every minute and request the passcode

My iPhone 6s was running fine last night. This morning when I started using it, it would reset about every minute and if I was doing something, it would stop that app and do a reset and prompt for passcode. I had to reset a new passcode off in order to use phone It still resets and I do have to enter passcode every time. i'm trying to download now the IOS 11.2. anyone know what is the problem and how can i fix it.