UV background missing

[micheus]

Not sure what happened, but if you say you already UV mapped the mesh and there is the proper material assigned to it, then in this case you can just reassign the correct image to the diffuse map of the material.
If you are in v2.1.* you can used drag and drop, if you are in lower version you can use Pick image (over the image name - Outline) and Drop picked image (over the material name).