The same exact thing happened to me this morning (and I'm a PLus-level subscriber). My most recent encrypted note with a list of important passwords completely disappeared when I accidentally hit the backspace when trying to enter the de-encryption password. As a result, the whole note text seems to have disappeared.
This is a serious design flaw in Evernote. At a bare minimum, all versions the program should provide a warning before erasing the note text, or--better--not delete the encrypted text at all.
(To be clear, I still have the note itself, but now it's blank, There's no copy of the deleted encrypted text in either the Evernote Trash or the computer's Trash files. Apparently, I have to upgrade to find past versions of notes, when this is clearly a design flaw!? Seems like lazy programming to me. Seriously considering abandoning Evernote--even though I've been a member since 2009.)