Hydrangeas that show signs of wilting indicates that something could be wrong with the shrub. The severity of the wilted leaves on the hydrangea depends on what's causing the shrub to wilt.
Mushroom root rot, also called Armillaria root rot, causes hydrangea leaves to wilt. Hydrangeas affected by this disease usually don't recover. Phytophthora root rot also causes hydrangeas to wilt. However, unlike mushroom root rot, hydrangeas have a better chance of recovery from this disease. Use an appropriate fungicide to treat phytophthora root rot.
Afternoon sun may cause hydrangeas to wilt. However, unlike wilting caused by plant disease, hydrangeas usually recover from weather-related wilt. As a precaution, confirm the soil is moist when you notice the wilted leaves. If it is, resist the urge to water, as this may contribute to root rot.
If the hydrangea is planted under a tree, it may be competing with the tree for nutrients found in the soil. This may cause leaves to appear yellow, as the tree takes up most of the soil's nutrients, leaving very few nutrients for the hydrangea. A soil test will reveal what types of nutrients should be added to the hydrangea's soil.