Email saved in folders disappeared- sender's name is there w/ 'undisclosed recipients' & no message. Why?

I have noticed that a few important emails that I've saved in folders have mysteriously disappeared. I've noticed this in a couple of folders - in the list of emails, the title and sender's name is listed, then when I go into the email there is no title, 'undisclosed recipients' and no message at all. These are emails from the month of Feb onwards. Does anyone know of a possible reason for this?