Hydrangeas will abort the blooms and flower buds if they become stressed. It is possible that when it was exposed to the sun, it was getting too much sun so the blooms dried out. While blooms can be exposed for the sun for a few hours, there is a point when the plant cannot provide enough moisture to the blooms and the sepals dry out. Limit their sun exposure to 2-4 hours only and then, only in the morning hours. This is more of a problem in the summer months. Once the blooms have browned out, they do not revert back to their original colors. Just deadhead them.
See https://www.hydrangeashydrangeas.com for more information about hydrangeas.