Since it's been three weeks, if they look mostly OK, they will probably start laying again eventually-- but it might take a while. If the dehydration triggered a molt, it could take up to a few months. (If they have lost a bunch of feathers and are growing new ones, that would be likely.) But since one is laying, the others might be close behind. The only way to really know is to just keep waiting.
So glad to see this posting on dehydrated chickens. Same thing happened here while on vacation for four days - the automatic drip developed a leak and the reservoir went dry. Six hens and it's been a week since and have found only two eggs. Now they have two water sources all the time just incase one goes dry. Thankfully, all chickens appear healthy and are eating heartily.
