I do not know if this is common / occurs frequently, but it has happened to me and a few other people. While making a resource pack, with importing default Minecraft assets, the default panorama in the title screen was just gray.
I checked the assets folder and the default panorama had been replaced with plain gray images. Once the images were deleted the default panorama was restored.
you need to delete (resorce pack name)\assets\minecraft\textures\gui\title\background