New e-mails are disappearing from my Inbox

I had a large number of recent e-mails disappear from my In-box without warning.  I also received some new e-mails today, opened one and responded to it (from a known source) and all e-mails received today, including the one I responded to disappeared from my Inbox.  These are work related correspondence, and it is critical that I have access to my e-mail.