Why are the leaves drooping?
There are a number of reasons why your plant's leaves might start to droop. Your plant's leaves and soft stems are held upright by cell turgor. The pressure of the cells exerted on each other keeps leaves firm and strong. However, when your plant is unhappy and its natural balance gets disrupted, this hydrostatic pressure is lost and its leaves become weak and floppy.
Symptoms
Droopy leaves
Floppy leaves
Dry or wet soil
Common causes
The most common cause is a watering imbalance - either overwatering or underwatering, but it could also be due to exposure to cold draft, too much light or intense heat.
When your plant is exposed to watering-related issues, and starts to droop, it's because your plant is not wicking up adequate water.
If your plant doesn't perk back up or if the soil is already damp or wet you'll need to further investigate what could be causing the drooping leaves. Damaged roots due to overwatering, being pot bound, poor drainage in the pot or soil, being over fertilized or being grown in cold conditions can all cause severe damage to the roots. If the roots are damaged, your plant will not be able to wick up water even if the soil is wet. Repotting and pruning off damaged roots are the best option in this case.
What to think about
If your plant has been shocked by temperature or intense light, move it to a more sheltered position and allow it to recover. If it's been severely damaged the injuries may continue to occur and develop so just be patience. When lots of foliage has been damaged you'll likely need to cut back a bit on watering your plant as it won't be able to drink as much as it used to
If the leaves are drooping and the soil is very dry, start by watering, or even showering your plant. It should perk back up within just a couple of hours but may take a few days to recover fully. Be careful if your plant has been allowed to dry out for an extended period of time, as it can easily get overwatered by a big deluge. If your plant has had a period of drought, water slowly to help it perk up without getting a big shock.
Note, though, that drooping leaves aren't always a sign of trouble! Some plants naturally start to droop with age - evergreens in particular can be prone to this. Other plants tend to have somewhat droopy-looking leaves even when they're totally healthy, especially when compared to the plump, firm leaves of succulent-type plants.
Try to learn what is 'normal' for your plants so that you can act quickly if something suddenly changes and they start to look unhappy. Taking regular pictures of your plants as part of the 'Progress Update' in Planta is a great way to keep track of any changes in your plant over time!