I just did a little exploring on google. Evidently hydrangeas, like many other plants, are prone to root rot when the soil is too moist. When they experience root rot, the plants tend to wilt, which to the gardener, re-inforces the notion that the plant is not getting enough water. The gardener then waters even more, and compounds the problem.
If I were you, would pull one of the plants and check to make sure that the roots are still healthy. If they are then simply let the plants dry more between waterings. If having difficulty deciding when to water, perhaps get a moisture meter probe. If the plants have developed black rotting roots, then IMO the best chance for recovery would be to pull the plants, shake most of the dirt from the roots, trim any unhealthy roots away and then repot the plants in nursery pots until they can recover. Also if lots of roots are lost and must be cut away, it would make sense to prune back a fair amount of the top growth, so that there is not too much demand on the roots.
I don't have specific knowledge relate to hydrangea, and the above advice for remediating the problem is based upon plant care in general. You may want to research your specific kind of hydrangeas and/or talk to a competent person at a nursery that sells hydrangeas. Sometimes early spring pruning can cause problems with some plants. In a plant with root rot you may have not other choice, but would be nice for corrective actions to be confirmed by an expert.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.