Did your phone get hit or dropped? The most probable reason for that seems to be related to the inner pressure point connector of the power button (located right at the motherboard). Take it anyways to a technician: He would probably tell you that it needs to replace the motherboard of your phone, as it the integrated connector it's difficult to solder again (or doesn't have the time/skill/part for it). Hope it works again! Narmith

That happened on various (different) phones of mine through my life. Consider doing a factory-reset state formatting, this could solve not only this issue, but make your phone faster (it "refreshes" the phone), but take precaution with your data (remove the SD card!) before that and copy all your contacts to your email provider account (meaning, do not have only one copy of the contact, as the formatting will erase your personal data on the phone), write them on an office excel sheet or paper if necessary. Also, this would erase all your downloaded and installed apps that didn't come on-default with your phone. Hope it serves you Narmith

MOST probably: It would seem the battery is damaged and needs to be replaced (did you dropped your phone?). Could you ask a friend for a temporary battery swap to check this, or buy a new one. Optional: Also, consider doing a factory state formatting, this could solve not only this issue, but make your phone faster (it "refreshes" the phone), but take precaution with your data (remove the SD card!) before that and copy all your contacts to your email provider account (meaning, do not have only one copy of the contact, as the formatting will erase your personal data on the phone), write them on an office excel sheet or paper if necessary. Hope it serves you. Narmith