Sounds like you killed the battery then, especially if it was over discharged for an extended period of time.
I have used various chargers with digital readouts that provide information on the battery voltage and that can be useful to understand what the minimum voltage is for the charger to provide current.
If you want to "hack" some charge into a dead pack, you can try wiring it in parallel with a fully charged pack to help level the voltage. Back in the 80's we used to charge our batteries with a mini jumper cable hooked directly up to a 12V automobile with a charge cable that looked like this:
View attachment 183237
Basically we would touch the NiCd battery every few minutes until the pack was hot to the touch and that was the indicator that it was fully charged. talk about low tech, ha!