In most cases, you can go change it back.
Here is how to do that for Chrome, Firefox and Safari:
Chrome:
In Chrome, go to the Chrome menu and choose Preferences.
Scroll down until you see Search and change your search engine in the selection box.
Close the preferences tab or window when done.
Firefox:
In Firefox, click on the Firefox menu and choose Preferences. Click on Search on the left side. Then select your search engine. Close the preferences tab or window when done.
Safari:
In Safari, click on the Safari menu and choose Preferences. Click on Search and select your search engine. Close the preferences window when done.