Also, look in System Preferences > Universal Access > Seeing
Check to see if Zoom got turned on. If it has, you can turn it off there.
Note that Zoom has a keyboard activate/deactivate command, Command-Option-8 which can allow accidental activation of Zoom. You can disable that keyboard shortcut if you wish. To do so, go to System Preferences > Keyboard > Keyboard Shortcuts, click Universal Access in the left pane, then uncheck the Zoom item in the right pane.