Here is a summary. The problem could be one of 4 things as most on here have alluded to:
1) bad LED (very unlikely)
2) bad driver (more likely than a bad LED)
3) no or poor connection between the battery and the driver (hopefully that is all it is).
4) dead battery
Assuming that a known good battery is being used, that leaves the most likely problem as #2 or #3. The way to eliminate #3 is to do as Beam suggested and directly connect a known good battery to the head or use a power supply set to 3.2 volts. If still no results then most likely the driver has failed. If all else fails wait until the new owner of Foursevens emerges and see if they will warrant it. If not, then someone on Ebay or CPF would likely buy it in its existing condition.
A good suitable replacement would be the Jetbeam Jet II MK or Fenix E15.
Just some thoughts
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