Thickbox hasn’t been updated in a while, and it is now incompatible with recent versions of jQuery (1.3+). What happens is that when you click the thickboxed element, the screen goes gray and the the loading “barber pole” image loads as expected, but then nothing happens… it just hangs on the loading image. The reason why is because Thickbox uses ‘@’ in jQuery, which is now deprecated.
To fix, simply open up the un-compressed thickbox.js file and go to line 79 and change the following:
TB_TempArray = $("a[@rel="+imageGroup+"]").get();
… into …
TB_TempArray = $("a[rel="+imageGroup+"]").get();
Once you make that change, Thickbox should work as expected!