How can i replace missing mesh in UV editor? I have a strange problem. I am doing the doughnut tutorial series, and things were going swimmingly until I some how lost part of the mesh in the UV editor. The screen shot below shows that I have a texture, and it's mapped to the object correctly, but a chunk in the middle somehow went missing or I accidentally deleted it. The renders now come out with black sections that matches the missing area in the UV edit. Is there a way to re-construct the missing mesh for UV editing ? I have tried unwrapping the mesh again, but this doesn't seem to have any affect restore a single and complete mesh.

--Update--
I've managed to select the missing mesh on the object, and attempted to unwrap it, but i can't make blender unwrap the selected mesh as 2d square.
Rather than fix the missing mesh, the best approach I found was to simply "reset" the unwrap, select all faces, then UV Unwrap again and use "Follow Active Quads". (hat-tip "moonboots"). I had to resize the mesh in the UV editor and align it onto the material. Other methods unwrap the selected mesh in ways that makes it unworkable to fill areas. When you initially unwrap a mesh, Blender (2.8) automagically lays out the mesh to match the boundaries of the material, but it appears that you can't repeat this process without manually aligning it.