Try to remember what you just did before the overlay problem appeared.
Have you installed / activated a new plugin?
It’s most likely you have a conflict with some of the plugins you have activated, so I suggest you check the plugins first.
Deactivate ALL plugins temporarily to see if this resolves the problem.
If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
If this solution doesn’t work, please give me an address to your website and your WP credentials in private reply (check the “Set as private reply” checkbox below the post) so I can check the problem by myself.