Is the Camera Button but you don't know how to use it, it have two time press, at the first click it take focus, when you push harder it take the pictures. If you cannot take photos then the button is faulty and you should go for replacement.
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My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)
How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4