Hi, and welcome to FineTunedMac.
For starters, we'll need to know with which browser and version you're having your problem.
Assuming it's Safari, try toggling Safari > View > Hide Toolbar on and off.
If that doesn't help, try Safari > View > Customize Toolbar, drag your address bar from its location 'til it goes "poof", and drag it back from the customization pane.
Hope that helps.
Edit: Your version of OS X may also be helpful.