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

[Willow Kitty90]

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.

 

[Laura Knotek]

Welcome to Android Central.

What phone do you have?

 

[Willow Kitty90]

Alcatel Onetouch A571VL