The address bar is a great way to quickly check out the current URL of the page your currently viewing. If you use a screen reader you can hit ALT+D to jump directly to it to copy a url or to type in a new one. I'm not sure how this happens, but now and then the address bar will disappear. Restoring it is pretty easy.
With Internet Explorer go to View, choose Toolbars, and click on Address Bar.
In Netscape navigator go to View, Show/Hide, then click Navigation Toolbar.
See, that wasn't so hard. Now you should have your address bar back.