First clue is that it happened when the live camera was unplugged. It would seem that the odds of it being a contact issue would be an astronomical coincidence. More likely a protocol issue or corruption when the system tried to switch over from tethered power to battery power ... especially if it was in the middle of doing something.
BTW, how did you update/reload the body firmware if the camera doesn't recognize back ... firmware updates for the body are done through the back via Phocus aren't they?
Try this set of fixes: No guarantee ... it's just the first thing my dealer taught me to do if I had any tethered issues ... which I haven't.
1) Remove the CF card, lens, prism, back and battery from the body. Press the shutter release button for a while to discharge any residual power. Charge a fresh battery to 100%.
2) In Applications, go to the Utilities Folder and open Disk Utility and run Permissions repair ... once done, run it again.
3) Make sure Phocus is closed, go to your user account then to Library, then to Preferences, alphabetically look for dk.hasselblad.phocus.plist and trash it. Empty the trash, and restart your computer.
4) Open Phocus.
5) Reassemble the camera including the fresh battery. No CF card. Use a different tether cable if you have one ... use the FW800 to 400 cord that came with the H4D/40, which will eliminate any FW800 issue on your computer as a cause. It should be noted that all of these digital backs require more power than most other firewire devices, and the H4D/40 draws more power than the H3D-II/39 backs did ... as others on GetDpi have mentioned, laptops and iMacs aren't all that great at providing power through the firewire ports, and a powered hub does help.
6) Start the camera, plug in the tether cord to the digital back and see if Phocus recognizes it. If not, try closing Phocus, and reopen it. If not, close Phocus, restart your computer, and reopen Phocus. Do not turn off or unplug the camera while doing this. Do not get impatient ... give it time to think.
Good luck,
-Marc