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