No guarantees, but if you take the phone in for service, the first thing that the tech will usually do is perform a Forced Restart on the phone. So, if you have not already done this, it is certainly worth a try.
Follow the instructions very carefully, as this is a bit tricky. You may need to repeat this several times.
Force restart iPhone - Apple Support
There are only 2 possible results when you perform a Forced Restart:
1) The phone returns to normal operation.....or.....
2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.
You can find a list of authorized service shops in your area using the link just below:
Find Locations