It could have been about 40% charged, which is what nimh is supposed to be stored at/around. Your best bet would be to run it until it "dumps" (goes flat), then recharge and re-check the numbers. It might take a cycle or two to wake up, too.
It's safe to charge the night before; I've charged my packs 3 days in advance, back when I used to run three classes (electric buggy and truck, 8th scale nitro), then peak them off the morning of the event.
Seeing as it's the night before, you should be just fine.