Gallery botton in the dashboard disappeared

A few days ago all of a sudden I could not open any option of the gallery menu any more. When I clicked on an option, there was always an empty white page shown. After updating NextGEN Gallery plugin and installing the external Reset Tool plugin, the Gallery botton in my dashboard disappeared and one of the photo sliders on our homepage has gone too! Our page is http://www.axa-arag-cup.ch. In the lost slider pics of our sponsor companies have been showed (each one for a few seconds). We urgently need to update (actualize) our homepage, and this in impossible without finding contents and options of the galleries. I am only a simple WP user and not a WP professional. So each kind of understandable help is very much appreciated!