Thank you for everyone's help.
Here's what I did to fix it:
1. I signed in to the web version and it was empty. So all three of my devices were empty.
NOTE: Earlier, when given the advice to sign out of one device, I should have been told that the one device I should keep is the WEB, because that's where the missing notes are likely to be if my phone and desktop are blank. In my case, all three were blank.
2. I signed up for a paid account for one month.
3. I opened a chat with an agent. He said I DID have two accounts. How could that be, since I was using sign in information I'd had for years?
Here's how it happened:
4. I figured out that I had started an account years ago with one email address. This is the one that I started adding all my notes to.
5. A short while later, I THOUGHT I changed the email on the account, but I had inadvertently started a new account. When I made the change, I wrote the new email address in a fresh password book.
6. As I continued to add to my notes and books, I was automatically signed in. I THOUGHT I it was signing me in with my newer email, but it was actually signing me in with my old email. This happened over the course of several years.
7. When the update happened recently, I was asked to sign in, so I pulled out my password book and used it to get in, but found nothing was there because I had never added a note to the account attached to the newer email. By this time, I had no memory of having the older email attached to the account, and I had password information for the newer email in my password book, so that was why I was so confused. I had two accounts, but had created them both years ago without knowing it.
8. I figured out how to sign in to my old account, and found my notes!
9. I updated it to the email address I thought I had all along.
10. Then I cancelled my subscription to the evernote paid version. So it cost me a few bucks to figure it out, but it was a lot faster way towards a solution.
11. I sent a message to management asking that they make it MORE OBVIOUS when people have more than one account. There are a lot of people having similar experiences to mine and it makes one feel unsafe to have their information disappear. Obviously, their "update" and their "change your email address" procedure is also flawed if so many people are finding themselves with new accounts. Something is not set up logically.
Thank you again for your help. I posted this follow up so others can find answers. I truly believed I did not have two accounts because it happened so long ago. So when people said "you probably have two accounts" I said "nope, I'm using the same email address and password that I set up years ago." Obviously I was wrong.
Thank you again.