I created a custom app in my account and immediately afterwards, all my files and folders have disappeared. I think they are still there somewhere, as the account page shows 18.3GB / 50GB. There are now several shared links (over 100) that no longer function due to this issue.
These files are more important than the oauth access. If the custom app (oauth) needs to be undone, I am okay with that.
There was a ticket submitted a year ago with similar issue, but, the fix he suggested (going to admin console and adding folders) did not work for me as my folders did not appear in the list.