Bob, given the fact that it now cannot be seen except by Disk Utility and DU wants to initialize it, I'd suspect that it suffered physical damage in the fall. Otherwise, it would be visible and functioning. It's unlikely that it simply somehow suffered directory damage with no associated physical damage as the result of the fall itself My bet is that DU will try, but fail to format it, especially if you've already tried TechTool and or Disk Warrior.
Of course, there is one possibility that doesn't necessarily mean physical damage - if the disk was being actively written to when it fell, it's possible that it was left in a state that rendered it unrecognizable. This is why we get the warning to eject drives properly before disconnecting them. If that's the case, then it may be possible to recover it by re-initializing, though like Jon, I don't know that I'd trust it.
