Every mobile device has its "PC Companion" software, whose purpose is repairing the smartphone's operating system, making backup of personal data, etc. For example, iPhones have iTunes, Sony mobile devices have Sony PC companion, etc.
Connect your device to the computer via a USB cable, (install and) run the software, follow the instructions by the software and wait for it to repair your device.
Note: If you don't have the software installed in your computer, you can download it from the manufacturer's website. Also, make sure that you make a backup of all your personal data, such as photos, videos, sounds and other documents.
If you tried all the other solutions, but none of them worked, reset your device to its factory settings. Before you reset it, make sure to make a back-up of all your personal data, such as pictures, videos, sounds, etc., because they will be deleted from your device.
See the don't know how to reset my device guide if you are not sure how to reset your device.
If you tried everything else to fix your device, but unsuccessfully, take your device to a repair store.
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