Today this happened to me whilst I was using Evernote (Mac 10.5.7) ie Not when I came back but whilst it was the active app.
I was reading notes, clicking on mini note headers in the list, when suddenly it stops showing content of the note, ie blank, I immediately returned to the note I was was displaying previously and the right note screen remained blank.
Might be relevant to programmers, as it was not the usual come back to the app from a time away, but stopped between moving from one note to the next.