If you have done what **Czarcasm **mentions above, pressing Ctrl+0 (zero) together on your keyboard will set everything back to the default zoom level. This works in Chrome, Firefox and IE.
Ctrl+ and Ctrl- (control plus and control minus) both do the same as control+scroll. I.e., zoom.
Perhaps, however, your browser became fullscreen and you can’t turn it off? If so, F11 is the button to toggle fullscreen in firefox. It’s probably the same in other browsers.