Thank you for the suggestions. I never did get the mesh to look the way it did before the problem occured. I think I have determined that using the cleanup tool (specifically "Edges with Zero Length" and "Faces with Zero Geometry") is the cause of the problem. "Unlocking Normals" in the normals menu went a long way towards correctiing the problem, maybe 75%. (Why were normals locked in the first place? Nothing I ever did.) But none of the other commands in that menu (alone or in combinations) returned the mesh to the pristine state it was in originally. Once again I had to return to any earlier version of the file, losing large quantities of work.

I simply don't use the "Remove Geometry" section of the cleanup tool anymore, unless I have specifically saved the file before doing so. Probably a good idea anyway. Thank god for revision control.