No Lightbox plugin working

Hi,
I just found a big problem on my site… I always had WP jQuery Lightbox working on my theme, but now it’s working no more. I tried a lot of lightbox plugins but no one worked…
I checked everything:
– no plugin conflict problem
– I have <?php wp_head(); ?> in my header.php
– I had no theme changes
– I didn’t update wordpress

Any idea? It’s very strange… Thank you!