The leaves on the picture look papery, which is a stage in which they can get before browning. Hopefully, the plant will react to the water and produce new foliage in 2-4 weeks. The root ball starts to repel water when it dries out too much so dunking in water for a very long time helps. You can also put a soaker hose in drip mode above the root ball but this is easier. Although not practical for a plant that has been planted in the ground.
Until it leafs out, maintain the soil as evenly moist as you can so the soil does not dry out then you water it, then it dries out again and so forth. Keeping it in morning sun or dappled sun will help it as long as it is not windy. You can also put it in bright but full shade all day (like underneath a tree). Hopefully, we caught it before it was too late.
To tell if the soil needs water, insert a finger into the soil to a depth of 2-4" (potted plants) or 4" (planted in the ground) and water if the soil feels dry or almost dry. You can probably do this daily, early in the mornings, at the same time for 2-3 weeks. After a while, you get an idea of how often to water. Maybe write notes in a wall calendar every time that you watered. Then after 2-3 weeks, review the notes and calculate on average, how often you were watering. Such as, every 3 days; every 4 days; etc.
Water only the soil, never the leaves from the base of the plant outwards in all directions. If planted in the ground, you would like a single watering to reach all the way down 8" and make the soil feel moist at that depth. In Spring, you can start with 1 gallon of water and tweak that amount if it is not enough. As temps regularly climb above 85F, hydrangeas become heat stressed and we have to increase the amt of water per watering (try 1.5 gallons). As temps regularly climb above 95F, increase the amt of water per watering again (try 2 gallons). Then reverse the process when temps go down. Tweak as needed for your conditions which may require even much, much more water than those numbers.
