Is she a goner?
I left a lantern running on very low level overnight. It runs on 3xAA NiMH cells. This morning it was still running, but when I pulled the cells out, two of them were at 1.0V and the third one was at negative 100mV. Charger wouldn’t charge it - I guess it thought I inserted it in reverse. 
This is a high capacity (2450 mAh) Amazon Basics. Toss it?
Try to “parallel” charge it first with another working battery for a short time (eg. try 1 minute first, check if the voltage increases).
(‘parallel’ charge, ie. connect a wire from + to + side of the battery, another wire from - to - side of battery.)
Maybe it may “activate’ the negative voltage battery, or maybe not. But more for testing purpose, I’ll probably not use it as ”regular use” if the battery does get recovered, not until doing a few cycles test first (ie. in case the battery does get recovered, try to charge-discharge a few cycles first)
That’s why married or matched batteries are important… when placed in series. On discharging, the voltage of batteries goes down. But when the quality/age of the batteries are different, it can happen that the better batteries still have a positive voltage, while the worst battery of the lot goes into reverse. Eg. is pushed into reverse by the more powerful ones in the string.
This does not happen in a 1-battery situation. Here the voltage sag stops a 0 volt.
I would say: dispose of it in a responsible way.
That’s why people advise to use protected LiION’s if the light has a 12V battery carrier. The protection functions on a 1 battery level. If the worst battery shuts down, the whole light shuts down.
Powerful lights often need more power than protected batteries can deliver. In that case marrying unprotected batteries can be a solution. Always use them together, always charge them together. Some people suggest to rotate the batteries after charging. So 1-2-3-4 becomes 2-3-4-1, becomes 3-4-1-2 and so on.
Parallel activating with a wire can be done to reset the protection of a battery that has tripped.
I would not suggest this on a battery whose chemical balance is thoroughly disturbed.
Improper or inadequate Cell Balancing can cause a voltage reversal in a series string when a pack is taken down to a too-low level.
A reversed cell is damaged beyond repair or recovery. If it were a Lithium cell it would be a very dangerous fire hazard; likely the same for NiMH.
Negative voltage is bad for NiMH but it doesn’t destroy them as much as it does for Li-Ion, you may be able to recover it.
Those three were married.
I let it rest a couple of hours, and the voltage bounced back to positive 0.9V. I am slowly charging it now just to see what happens.
I’ve had a number of NiMH batteries go down to 0 volts. All were fine after I cycled them several times and I can’t distinguish them from my other batteries when doing long-term discharge testing, etc
I`v had that happen with a few D cells in the past, after cycling them a few times individually they worked as normal again, but 2 of them would never seem to charge fully after that?
Recovered NiMh more than a couple times. Not good for the battery. Depending on initial quality, age, use, and how badly/long it was ‘run reverse polarity’ will ultimately determine the damage.
Dumb charger may boost it to ‘smart charger’ level. If not, parallel charging likely will. If that doesn’t work, then I would toss it.
If it recovers, mark it somehow, then just keep and eye on it. I put a big read D on it signifying ‘dead’. Sometimes the ‘D’s’ accumulate. 
Never had any recovered cell do anything disastrous, and I’ve been reviving them when occasionally needed for near 2 decades. Some have been almost no different from others purchased at the same time. Others have lived a shorter life with reduced ability. I do try to match cells of similar ability and use them in appropriate situations.
I cannot recommend this for lithium.
I have an old Gateway 2000 RF wireless keyboard and mouse that I still use and ditched the alkaleaks early on when my little bro and SIL gave me a gift card to Rat Shack.
I went down and bought their Rayovac IC3 AAs and AAAs and the 15 minute charger. They worked great, but that charger really hammered the batteries at times.
I didn’t know about ‘reverse charging,’ but I often scratched my head over the negative voltage readings I often got, LOL.
Chris
Reverse-charging is pretty bad for NiMH (and NiCd). Had a few go that way, long before I had analysing chargers.
Today, I’d try charging it to see if it holds, and if so, give it a few cycles (“refresh” function does that nicely), and see if it tracks what the other cells are doing.
Else, it’ll always be the runt of the litter and will just make the problem worse, always being the first one to hit zero and then negative when drawn down too much. Soon, the others will still be at 1.2V when it hits zero.
Lightbringer has a point. That’s why the cells get marked. That way I can track if they are holding up, or are continuing to fail.
If they are not performing, but still working ‘well enough’, they get downgraded.
If they are not performing up to my expectations > time to recycle.
That`s what I did with my 2 that wouldn`t fully charge, turned them into a nightlight, they`de last all night with a little 300ma incan bulb.