Hydrangeas typically droop because of issues related to water and environmental stress. The main reasons include:

• Underwatering : Hydrangeas love moisture, and when soil dries out, they lose turgor pressure causing leaves and stems to droop. They need deep, consistent watering, especially in hot weather, and mulch to retain moisture in the soil.

• Overwatering : Too much water can suffocate roots, leading to root rot, which also causes drooping and sometimes yellow or brown leaves. The soil should be allowed to dry at least a bit between watering.

• Poor soil drainage : Heavy or compacted soil like clay holds too much water, and improving drainage with organic matter or sand can help.

• Excessive sun exposure and heat stress : Too much direct sun, especially in the afternoon, can cause rapid water loss and drooping. Providing shade in the hottest part of the day helps.

• Too much nitrogen : Excess fertilizer can lead to weak, fast growth and heavy blooms that droop because the stems can't support them. Balanced fertilization is recommended.

• Heavy blooms and lack of support : Large, heavy flower heads on hydrangeas can weigh stems down, especially if they are weak. Staking or pruning flower buds can provide support.

In summary, the drooping is mostly due to water stress (either too little or too much), environmental heat or sun exposure, or physical factors like heavy blooms without support. Checking soil moisture and texture, watering appropriately, providing shade when needed, and managing fertilization can help revive and maintain healthy hydrangeas.