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Start with a Soft Reset. Hold the Volume Down + Power buttons until the phone vibrates and reboots. About 10 seconds. You will not lose data.
If the problem persists it just might be a corrupted SD card. Power off and remove the card, then boot back up.
If all else fails try to flash a clean copy of the OS by using the second tool on this page. How can I recover/reset/restore my phone software? - Microsoft - USA This will completely reset your phone, but you can restore you last backup during setup when you sign into the same Microsoft Account you used before.
If it is still a problem then it is hardware and you need a repair shop or warranty replacement.