OK, the problem appears to be solved. These are the steps I took:
Log out of the Evernote desktop client (VERY IMPORTANT - don't just uninstall it).
A dialog will pop up, asking if you want to keep or delete your local data. DELETE IT.
Uninstall Evernote.
This step involves a third-party utility (either Hazel (by Noodlesoft) or AppCleaner (free)) and may or may not be critical, but I recommend it just in case: in Hazel, if App Sweep is enabled, you'll see a Hazel dialog asking you if you want to trash all the Evernote-related files and folders that have been left behind. TRASH THEM. If using AppCleaner, drop the app onto it.
Restart your machine.
Reinstall Evernote and log in. Hopefully you'll have your notes back. This could take some time.
HTH
