Where are you? And what has the weather been like?

That looks like cold damage.

We are in Ohio. The weather has been oscillating between warm and cold lately.

Late spring cold can definitely do a number on bigleaf hydrangeas, especially in some place like Ohio!!

Oh, man! Late frosts will do that to you. If you hear late frost warnings from the weather service, take action and water deeply the night before. Gee, I feel weird saying that though, Kelsey. We have been hitting the low 90s for highs lately! From someone who spent the day pulling weeds, sweating, inhaling pollen and is now sneezing -Kerchoo!- big time, enjoy those cool evening temperatures and send unused cold temps this way! ;o)) Ha!

The plant will now decide which leaves to brown out completely and where to develop new foliage. It may take 2-4 weeks or so though. Just maintain the soil as evenly moist as you can.

When the flower buds open (they look like small broccoli heads), if you see brown areas, that means that those parts got zapped by the frost too.