Which bit has come off ? The water inlet valve is at the back?
Copy and past the following in to your browser - http://www.bosch-home.co.uk/store/spareparts/Dishwasher/SMS6012GB/14 -- then spare parts list and diagrams. Page 1. This gives you the breakdown of the door area. Now fit the facia panel on first without the bottom panel and get everything in place. Once this is done put on the bottom panel and you're done.