If there is Warranty or AppleCare make Genius reservation or set up Service and take or send to Apple for resolution. If no coverage, you can use virtual Lock Button. Tap Settings App > General > Assessibility > Assistive Touch > ON > tap new white screen button > Device > Lock Screen. This can even be used to turn OFF iPhone, by holding down on it. To turn iPhone ON, connect to power, charger or USB of computer.