My uncle called me because he put his phone in his pocket and then lost his background picture on his Android phone. I first recommended he set another picture to see what would happen. Go to your “Gallery” select a picture and on the three-dot menu, tap and look for the option to set as wallpaper.
This didn’t work. I told him to restart his phone which didn’t work.
It finally occurred to me what might have happened. I told him to swipe down from the top of his screen – look for the power mode button – if it’s blue – click on it and when it finishes running it should be back to normal. That solved it.
He accidentally set his phone to power saver mode. Read here: How To Extend Your Battery Life Charge Android