Did you accidentally hide someone on Facebook that you want to see updates from? For a few seconds after you hide someone, you have the option to undo the hide. After that, you must modify your wall options.
You can unhide friends on Facebook by scrolling all the way down the wall until you see the Edit Options link on the right side of the middle column. Clicking on Edit Options will give you a menu allowing you to unhide friends or applications.
To unhide a friend, simply locate their name in the list and click the "Add to News Feed" button.
To unhide an application, click at the top where it says "applications", locate the application you want to unhide and click the "Add to News Feed" button.