Back button not working

Hello everyone.
So i dont know why but suddenly the back button stopped responding ENTIRELY. It just happened unexpectedly. I`ve tried remapping buttons with some root apps but no app ever recognised the button. I have even tried going to recovery (TWRP), but it doesnt work there neither. It is like the phone doesnt have a touch back button at all. But home button works flawlessly. Any ideas or suggestions?

maybe try install stock software ?

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