That WAS some heavy rain. This is a common problem with hydrangeas and peonies. There’s not much that can be done other that providing support for the branches with the metal stakes with rings at the top to hold the stem upright. You should feed it every spring with Espoma Flower-Tone and that can make it as sturdy as possible but, when we get a pounding 3″ of rain in a couple of hours, your hydrangeas are going to flop until they dry out.