iPhone DEAD after overheating !

Did you get the temperature warning before that?

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

To find out more about the damage and your service options, visit an Authorized Apple Service Provider next to your place, or make an appointment at an Apple Store.

You can also send it in for service by contacting Apple Support about that. In some cases, the carrier can also take care of that problem.

Find an Apple Authorized Service Provider
Make an Apple Store appointment
Contact your carrier
Back up your iPhone and get ready for service

Have any parts of the phone been replaced recently, for example the battery?

Where you using Wi-Fi or Bluetooth connections for a prolonged time?