Camera and flash light

Did you already try to set the phone back to factory settings, using your computer and setting it up again, without using the backup?

Restore your iPhone, iPad or iPod touch to factory settings

If the issues are not solved after that, the phone has a hardware failure, and has to be serviced by Apple or an Authorized Apple Service Provider next to your place.

Apple - Support - iPhone Repair and Service

Did you set it up as new device, without using the backup for those 15mins?

If yes, contact Apple Support to have it diagnosed. They can run an online diagnosis, maybe the error logs contain information about that failure.

iPhone Support Apple

In that case, you should contact Apple Support directly and ask them to diagnose your device, or visit an Authorized Apple Service Provider next to your place, to do so.

Have any parts of the hardware been replaced lately, for example the battery or the display?

Has the phone been dropped?