Now it's behaving differently. Only some notes cannot be edited. (Both web and desktop). It looks like it has effected only my most recently edited notes. In one such note, the information appears in the preview pane, but if you choose "open note" from the context menu, the note opens with no text at all, not even the text that appears in the preview.
In other notes, you cannot get an active cursor to appear within the note, no matter how you click on the note or request to "edit" it from the drop-down menu. It's like you're clicking on nothing at all and the note can't be edited.