A NiMH or NiCd cell reading of zero volts usually indicates an internal
short, caused by metallic dendrite growth. If this is the problem with
the cells, they won't respond to any ordinary attempt to charge them.
It's sometimes possible to "zap" a shorted cell (by discharging a very
large capacitor into the cell) to burn out the shorts, but you end up
with a cell that has higher than normal self-discharge and a tendency to
grow shorts again. So if they don't respond to normal charging, I
recommend tossing them and getting some new cells.

If the device has a charger designed for NiMH cells, that is, one that
charges at a fairly high rate and properly detects the end of charge,
then I highly recommend one of the newer low-self discharge cells such
as the Sanyo Eneloop. If it doesn't have a proper NiMH charger and just
pumps in a constant current charge without end of charge detection, you
should use NiCd cells because they can tolerate that charge regimen much
better than NiMH cells can. I've had very good luck with Sanyo and
Panasonic NiCd cells. Don't be suckered by inflated capacity claims
which the minor brands liberally use. Even with the better brands, the
highest capacity cells have historically had more problems with overly
fast self discharge and premature cell failure.

Roy Lewallen, W7EL