Use Assistive Touch until you can have the iPhone evaluated for Warranty Coverage fix.
Settings App>General>Accessibility>Assistive Touch>Turn it ON.
Use the new screen White Button, select Device>touch Lock Screen, this will turn OFF Screen.
If you touch and hold the Lock Screen button for 3 seconds it will give you the "slide to power off" slider so you can turn OFF your iPhone. To turn iPhone back ON, connect to Wall Charger or USB.