We had a problem with chicken feed up here... one particular brand... took roughly two weeks before people started losing birds. One friend got 4 quail from me, and started feeding it... two weeks later one acted really odd and keeled over dead... the other three started acting weird after that. When I asked her about the feed, and told her to switch, she did and they pulled out of it within a couple of days. It really could be the feed.
Coturnix do fairly well in the cold, so unless you moved them from a 65°F house out to -10F outside, I don't think it's the cold. If they were badly startled, they may have all flushed and hit their heads hard on the top of the cage and killed themselves that way... it happens.
Best thing is to do a necropsy, see if their necks are broken or if there's anything odd in their carcasses. Or send a bird to your state vet for testing.
I'm sorry for your loss...