On June 23rd, Facebook is deprecating (i.e. turning off) a number of plugins including the Like Box, Activity Feed, Facepile and Recommendations Feed. Not to worry, there are new plugins to take their place.
The closest equivalent to the Like Box is the new Page Plugin. It has the same features (face display, recent posts, share and like buttons) and a more streamlined look that includes your page cover image.
To learn more about the new plugins and how to install them on your site, visit https://developers.facebook.com/docs/plugins. Just do it before June 23rd.