Touchscreen
My touchscreen has stopped working all of a sudden. Could my charger overheated it? How much would it cost to be fixed?
iPhone 6
The charger cannot overheat the screen. If there is a problem with the screen, it can be addressed at the Genius Bar. You can start by trying the troubleshooting in this support document. If the screen on your iPhone, iPad, or iPod touch doesn't respond to touch - Apple Support
If that does not help, then make an appointment at the Genius Bar of the nearest Apple Store to have the device examined.