How to make a collapsible fieldset in my theme?
Your theme might include jQuery 1.9. With this, I observed in Drupal 7.34 that fieldsets would not work (always shown collapsed, unclickable). Also CKEditor would not load (using the ckeditor module) and there might be other JavaScript related misbehavior.

To solve this, use jQuery 1.8 instead. I recommend the module jquery_update to handle jQuery versions in Drupal.