Unless it was a very hard frost, I doubt the frost was the cause. It sounds like disease, but as RG said, it's not too common for the chard to get disease.
I think that DDF may be onto something when he asked if you used any unfinished compost. What happens is as the bacteria are finishing your unfinished compost, they "gas-off" in the soil and that can harm your plants at the root-level.
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