If holding the power button for 10 seconds to shut it down and clear the cache doesn't work it's probably a hardware issue.
Have you unplugged it for 30 seconds?
Also can you connect to WiFi?
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=== Update (08/07/2017) ===
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Sorry I hadn't been following this post more closely. Many times the WiFi/Bluetooth board stops working. WiFi modules are pretty cheap on Amazon and eBay. They are very easy to replace. Can't sync if the xbox can't see the controller.
Did you get it fixed?
Could be ribbon cable, or RF board.
If holding the power button for 10 seconds to shut it down and clear the cache doesn't work it's probably a hardware issue.
Have you unplugged it for 30 seconds?
Also can you connect to WiFi?