If you open the epub in Sigil, it will recreate the container.xml file in the META-INF directory in the file when it is saved. Calibre's editor errors out trying to open the file.
Another good reason to keep both editors around.
At the same time, error check the epub. I'd recommend downloading the epubcheck plugin for both editors.
Of course, there is a good chance the epub file is totally gorched. Time to get a new copy.