Internet Explorer can run in full screen mode, which is a great thing if you want to show a demo of a web site, but a big concern when all your navigation is gone and you don't know why. In full screen mode, all your toolbars and navigation fields will be gone, in addition to your start menu at the bottom of your screen.
If one day you find your browser like this, don't worry, you can get back to normal by hitting the "F11" key. This toggles you in and out of full screen mode.
You may have hit the F11key by mistake, or in some cases, a web based program may automatically set your browser in full screen mode (such has if your kids play a game like MineCraft on your system, which happened to me). It's quick to restore things back to normal with F11. Full screen mode may also be something you want to try for demos or other presentations.
While we're on the subject, when you get your tool bars back, you can also customize what you see. Right-click your mouse in the grey area on your toolbar and you'll get the choice to add a "Favorites" bar and "Google Toolbar", which are useful. You can pick and choose how much screen real estate you use for navigation.