When a Nikon SB600 is dropped and no longer turns on, it is probably because the inductor has been broken, as in: http://www.nslms.com/2009/09/07/like...sh-in-the-pan/

The battery assembly free-floats in the case and has a nice little tight cutout area that fits just over the inductor. So if it is full of batteries and shocked, this inductor can easily be sheared and broken.

All is not lost- I just fixed mine for under $25 - but that's because I ordered the wrong size inductor to begin with and also ordered a copy of the repair manual and parts list off eBay. I've posted details of the repair at the link above, although it is not for the unequipped or faint of heart...