Need header and chapter breaks for MS Word novel manuscript

I have a complete manuscript in which the header disappeared and the chapter breaks became irregular. I can get the header to appear, but I cannot modify the document when the header is visible. Nor can I fix the chapter breaks.

This started when I sent the manuscript to an editor and he returned it with the formatting trashed. Now the formatting on ALL my MS Word docs has disappeared.

I am a professional writer, seven books in print, but not very tech savvy. This is halted my work.

I use a Mac. He's on a PC.