Why are contents of folder in e-mail disappearing?

Could be for one of many reasons, but difficult to determine without more information. some things to consider:

1. You are inadvertantly deleting them
2. You are using an IMAP account and deleting them via another device
3. You Mail display settings are limiting what you can see (Settings App > Mail,Contacts,Calendars > Show).

Today I checked a few of my folders. I saw my saved emails for an instant then they disappeared and a message says no mail. Same thing happens to other folders. Afraid to open any more. I am not inadvertently deleting them.