It looks like at some point in time you added something to your website (additional plugin, theme change, etc) that wasn’t coded properly.
Looking through your html it looks like the culprit is the comment validation plugin. Basically instead of loading jQuery by using the WordPress provided copy, it’s attempting to load it using it’s own copy. Since jQuery is already loaded, and this new copy isn’t set up correctly, you get a conflict and as a result, slimbox2 doesn’t work.
I’d recommend telling the comment validation author to fix his plugin, though an alternative would be to comment out his line of code that loads jQuery.