Getting the error "$().once is not a function" till today noon my site working fine but after that suddenly my site was struck and in error console it give me below error.


  Error: TypeError: $(...).once is not a function Source File: modules/overlay/overlay-child.js?mixmsl on Line: 111.


/**
 * Modify forms depending on their relation to the overlay.
 *
 * By default, forms are assumed to keep the flow in the overlay. Thus their
 * action attribute get a ?render=overlay suffix.
 */
Drupal.overlayChild.behaviors.parseForms = function (context, settings) {
  $('form', context).once('overlay', function () {
    // Obtain the action attribute of the form.
    var action = $(this).attr('action');
    // Keep internal forms in the overlay.
    if (action == undefined || (action.indexOf('http') != 0 &amp;&amp; action.indexOf('https') != 0)) {
      action += (action.indexOf('?') &gt; -1 ? '&amp;' : '?') + 'render=overlay';
      $(this).attr('action', action);
    }
    // Submit external forms into a new window.
    else {
      $(this).attr('target', '_new');
    }
  });
};
The standard $ alias is not used in Drupal 7, to provide the best possible support for no-conflict jQuery.

You need to wrap your code in a closure like so:

(function($) {
  // Add code using $ as an alias for jQuery here...
})(jQuery);


See Managing Javascript in Drupal 7 for complete examples and explanations.