This is a Pages v5 package folder document which is the only format that Pages v5 - v5.2.2., and v5.5.1 will write out. By default Pages v5.5.2 and later default to writing single file format documents, which are not package folders. These can travel.
As long as package folder documents are saved to your local Apple HFS+ formatted hard drive, or to iCloud (Drive), the Finder will present these to you (and Pages) as a normal document, not a directory. If these package folders get damaged, then they may only appear as a directory, and become totally useless for Pages work.
As you say that you have been working on the same document for years, then it appears that you brought a document in from another word processor, and it was inherently converted to Pages v5 document architecture.
On your Desktop, create a new folder. Name it test.pages. It will question your sanity about giving it a .pages extension. Do so anyway. You now have an empty Pages package folder. Right click on this folder, and choose Show Package Contents. Copy all of the contents from the folder that you have shown, into this new, open test.pages package folder. Click the red traffic light in the title bar on test.pages. See if you can open it in Pages v5 now.