I've lost pages that way when I have more than one screen from the same program open (like two spreadsheets, pdfs, or Word documents). You have to minimize one to find the other one beneath it, then bring it back up using the dock. If the apps are not in TRUE full screen, then they appear as part of the desktop screen and you can't toggle between them. I like to toggle between screens using a four finger swiping motion on the magic trackpad (the way you do on an Ipad), or alternately, press command ⌘+TAB. When you resize a window it takes over the desktop rather than becoming a separate pane.
The problem with the suggestions people are offering is that they are NOT full screen mode.
