My phone keeps going black and resetting to default wallpaper. How do I fix this?

Apr 19, 2017
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For almost a week now whenever I use my phone or play a video, the screen goes black, goes back to the lockscreen, and resets the wallpaper to the default stock wallpaper. It hasn't done this for a month and started doing this again recently. It does it pretty much every minute I use my phone. I'm running android lollipop and it says it's up to date.