i had the same issue suddenly happen to me. i rebooted the appletv, and it never came back - simply an apple logo on screen, and flashing amber light.
reading another thread, it said about the same sort of thing happening with other apple [networked] devices when they couldn't find or use the network. the link is here:
http://forums.whirlpool.net.au/forum-replies-archive.cfm/1018461.html
so, with the apple logo on screen, i tried to ssh to it (i am a very naughty boy and have jailbroken the unit, with ssh, afp, usb support and the most marvellous XBMC application to help play the files the standard unit won't. you must do this immediately, or you will waste this really nice product)
i was surprised i could ssh to it at this stage of the [hung] boot process. the last thing i had tried to do was to mount some network drives using afp. doing a quick "ps auxwww" showed that my shell script with the afp commands was still there, after a reboot!! and the 1st afp command too. a "sudo kill" of the processes, and the unit immediately started the startup movie and voila!
hope this may help someone too...