Water them again.
They bounced back with the first flush of "free" water, but the substrate is probably still overly dry.
Once the soil dries right out it can become "hydrophobic", meaning water will flow through the pot but not really get absorbed.
In this situation, I found the best remedy is to give small amounts of water every hour or so until the soil plumps up again.
This may take 4-8 small waterings.
Root rot in very dry soil is almost impossible.