Power button stopped working

@sakib65793 most likely a power button flex situation. Since iFixit does not yet have a specific guide for this, use something like this video to get into it. The power button flex is readily available at many online places. You can of course also try the steps that are listed on the comments on this video

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!