"Add content" not showing content types anymore.

First check you can add the content types for example from
/node/add/article
(assuming you have a content type 'Article').

If you can add an article that way, you know this is a menu problem (by which I mean a problem with the menu system in the database). There is a major four-year-old Drupal bug which has never been solved AFAIK, but you might find some help for it in the issue on the topic http://drupal.org/node/550254. Just creating a new menu item is a possible workaround, but creating a new menu item in the Admin menu I have found not a stable solution.

I have found the solution of this problem -
Open the node.pages.inc file (you will find this in modules/node directory ). Replace the function node_add_page with the new (I have pasted below)

function node_add_page() {
  $content = array();
  // Only use node types the user has access to
  foreach (node_type_get_types() as $type) {
    if (node_access('create', $type->type)) {
      $type_url_str = str_replace('_', '-', $type->type);
      $content[] = array(
        'title' => $type->name,
        'href' => 'node/add/'.$type_url_str,
        'description' => $type->description,
        'localized_options' => array(),
      );
    }
  }
   // Bypass the node/add listing if only one content type is available.
   if (count($content) == 1) {


    $type = array_shift($content);
    drupal_goto($type['href']);
   }
   return theme('node_add_list', array('content' => $content));
 }