I’ve worked in Scrivener for 10 years and have NEVER had a problem with corrupted files, until today. 
I’m on MAC running High Sierra 10.13.6,; and Scrivener Version 3.1.5,
When I open my project i get an error (picture attached) saying I don’t have a binder.scrivproj file. I’ve done everything the error suggests. In my most recent backup (I back up on close), that too does not have that file. So, I have to go back to the day before - which means I’ve lost a whole day’s work.
Is there any way to get back that day’s work - or do I just have to suck it up and move on? Regardless, I will be changing my backup routine! 
Thanks in advance,
I fixed it. Posting my solution here in case it might help someone else.
My files are stored on my hard drive but synced to dropbox. When I tried to open the scriv file - I got the error. When I looked into the scriv package on my hard drive it did not include the binder.scrivproj.
So I went directly into Dropbox on the web. Downloaded the same backup version that I was trying to use on my hard drive. After download, I unzipped it and opened the SCRIV file. It worked.
Don’t know why the error said it was an older format, because it wasn’t. Regardless, downloading the backup file directly from DropBox and not using the synced file on my hard drive - seems to have fixed the problem.
Thanks everyone,
Dee