Once I had the same problem - update process finishes "succesfully" and navi asked to reboot..... I pressed Ok..... till now this navi unit is not alive! :-( But in my case I know the reason...... I had "highly customized" $;-) navi soft! So in my case only reflashing by spacial equipment is seems to be helpfull......

So if you are not sure in the quality of your naviSW - make your oun disk and try to update.......