This problem started in stages. 

At first my phone was occasionally failing to sync. Then I noticed two things added to my Phone App. One was an existing notebook had a new cover, and the other was a new Note Card with an odd text in it.

These were only visible on the Android app.

I changed the cover back to what it should be, and deleted the note.

Then I discovered that on the PC app there was now only five notebooks instead of the six there should be.

I looked on the web app, and there was still only 5 notebooks. 

The missing notebook is called Current, and has a number of text notes I update regularly. I can still see and update notes in the Current notebook on my phone, but the whole notebook is not visible on the PC app or the Web.

I don't know what to do now?

Thanks.