First of all, stay calm.
Such a situation can occur when the EPUB file has lost its "ebookness", so the system detects that it is a ZIP archive and Mac treats it as a folder.
As long as the files are physically in that folder then there is a good chance of success.
First of all, install the
FolderInv030.zip plugin
You run the plugin through the Plugins>Input>FolderIn menu
The "Select Folder to Input into Sigil" window will open and there point to your folder. After a while the epub file should load into Sigil.