desktop wallpaper turned black

Hi madelene93,

As we understand it, your Mac wallpaper turned black and when you try to set a different wallpaper, you get the same result. Is that right?

If so, we'd like to have you take two steps to help narrow this down. First, boot into safe mode. This performs a basic disk check and then prevents some items from loading on startup, so it could help rule out possible causes. For your Mac model, you'll use the steps below. How to use safe mode on your Mac - Apple Support

"Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window."

Once you start up in safe mode, see if you can set a wallpaper and then click Apple menu () > Restart.

You can also test in a newly created administrator account, which will help work out if this issue is system-wide or user-account specific. Check out this link for the exact steps to create a new account: Set up users, guests, and groups on Mac - Apple Support

Then sign in to the new user account and see how it goes when you try changing your wallpaper.

Let us know the results, and have a good day.