Most batteries really don't like being completely discharged - their ideal life is to be plugged in to a trickle charger and kept full until they fossilise
Something you COULD try if you're daring:
- Put the batteries in the fridge for a few hours.
- While you wait, get some jump leads and attach them to your car battery.
- Take the cold battery pack to the car.
- For the merest fraction of a second, connect the battery pack to the jump leads.
The idea is that the sudden surge from thr car battery 'shocks' the thingies off the Nicad wotsits, and thus restores them to as-new condition. Putting it in the fridge first cools them down to aid conducivity.
It's something I heard a while back, but never tried. So no idea if it works, fails, or makes the Nicads explode. But if it's kill-or-cure...
Failing that, get four new C-cell Nicads, cut open the current pack to see how they're wired up (dead simple) and solder the wires appropriately to the new batts. Hey presto, a new pack!