You could prune them now to eliminate the heavy blooms that are causing the problem. These stems are now weak but should get stronger in future years. And yes, you could use stakes as well. I would try putting chicken wire around the whole shrub and use stakes on particularly weak stems.

Were you able to determine if the problem was caused by heavy winds or by too much rainfall on the blooms?

Thank you Luis. The chicken wire sounds like a great idea. Yes, the collapse is definitely from the rain. This mophead hydrangea is about 25 years old. It has tons of beautiful blooms. Is there anything I can do to help strengthen the stems for next year?