Lumia 635 phone's Power/Lock key not working

[Windows Central Question]

Hi, I have had this phone for exactly 1 year and 6 months. Everything works except for its Power/Lock button. I have no problems operating the phone if it's not in power saver mode but I cannot pick up calls or do anything else once the screen enters power saver mode and goes dark. Moreover, if the phone is not in power saver mode (in active display) cannot get the phone to enter power saver mode by pressing the Power/Lock button, I. If I continue to press the button, the phone prompts me to scroll down to turn it off.