I've encountered this several times so far in a (v2.8) project of mine. I'll be going into the UV Editor for an object to get the mesh just right, and then once I pop over to Texture paint, I can't see it in the viewport. If I randomly paint around in the empty viewport, I can see that it is taking effect on the texture so the object is there, but hidden and I can't seem to un-hide it. I can't see what I'm painting on. What am I doing that's resulting in the meshes being hidden between UV Editing and Texture Painting, and how do I display the object in Texture Paint so I don't have to John Bramblitt my way through painting?
This one took an obnoxious amount of time to figure out how to get it to reappear, and even then I'm unable to replicate the problem of getting it to disappear now. If I can replicate the problem, I'll update my question further.
All it took to get the item to reappear was to go into local view by pressing / on the numpad to get it to re-appear. The / on the Numpad toggles Local View.
Local view isolates the selected object or objects, so that they are the only ones visible in the viewport.Source: The official Blender Documentation version 2.92