Sitemap Disappeared: Lost Work & Need Solutions

my duplicated site map suddenly disappeared on me and I spent a tremendous amount of work on it and now I have to start over. I can’t hit undo and get it back. I have the original site map, then made a duplicate to build off it. I attempted to delete a section on a page in the sitemap and everything disappeared - like flickered and only the original remained. I have to start over. It seems this issue has occurred for others in the past. Can’t find the solution to this. But it made me lose confidence in my work being saved.