As Stewie suggests, the Address Bar can be dragged off the screen, and if the "lock toolbars" choice is checked you can't drag it back.

In IE6, and possibly earlier versions, there is also an option to Hide Address Bar. Unfortunately, the article I found that tells you how to hide it doesn't mention how to unhide it.

You need to know specifically what version of Internet Explorer you're using. To find that, click "Help" and then "About Internet Explorer."

For Internet Explorer 6 ONLY, there's a Microsoft Knowledge Base article:

The Internet Explorer Address bar is missing KB842903.

The article suggests a fairly simple change in settings, but I don't have an IE6 running and the items listed that you should click are not present in my IE7. Instructions to include unlock (uncheck) the Lock Toolbars option, and if the Address bar was just dragged off somewhere it should come back at least to where you can drag it back from one edge or another of the screen.

If the changes in settings don't repair the situation for you, the article recommends deleting a line in the registry.

I'd recommend that you find the article, print it so it's in front of you, and follow the instructions pretty carefully. Using regedit isn't all that difficult, but there are lots of things that look almost alike and you need to be careful that you find exactly the right place to do what's needed.

I don't find an option to hide the Address Bar in IE7, but that doesn't mean it can't be done. You'd likely have noticed if IE7 was downloaded by Automatic Updates or otherwise, since IE7 looks quite a bit different than IE6 and earlier.

John