I am using QGis (latest version, OS X) and whan I create a map on map composer everything works fine. I export the map to pdf file and also goes fine. But when I save and close it and then close the main project and then open it again and then open the map composer that I saved, that map does not show anymore. However, I can still export the map but when it comes to editing I can't because everything is white. The commamd history appears empty. When opened on another computer (PC version) with a different version of QGis works fine. Here is the printscreen of my issue.
You probably have rendering set to Rectangle. It can be changed in map properties. Select your map item and in right panel go to Item properties --> Main properties there choose Render and hit Update preview.