It is difficult to diagnose the problem based on just the sudden dying. It may be that the soil is too dry or more likely is being kept too soggy wet which is deadly to those plants. It could also be that you have a root rot organism in there affecting them or perhasp that a weed and feed lawn fertilizer got on their root zones.
I suggest taking some photos and/or samples from the plants to your local County Extension Office to see if they can narrow the diagnosis a little more.
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