What happened to my iphone calendar?

What happened? this a.m. I checked calendar for an entry, everything was O.K. - Later I checked to add another entry and ALL entries were gone, everything was erased! - calendar now looks as though it has never been used. This is 2nd time this has happened. First time everything erased when I connected to my MacBook Pro. What's wrong??? I use my iphone calendar as my primary app't and reminder calendar and c/n afford to have this happen again. Thanks for the help. - R -