@tagessefikre The power button is located in the rear cover.
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That power button connects to the phone via a very "flimsy" ribbon cable. It is entirely possible that this cable is either torn or that the power button itself is no longer functioning. I would start my repair with replacing that button.
Use something like [https://www.youtube.com/watch?v=ProtTK_zMcg|this video] for the repair. For a replacement power button , do an online search. it is readily available on many places for not a lot of money.
While you work on your power button, 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 the same problem 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!