A drooping leaf is the least useful signal a plant gives you, because almost everything that can go wrong produces one.
Thirsty droops. Drowning droops. Cold droops. Root-bound droops. A plant that was moved across the room yesterday droops. They look close enough to identical that most people guess, and the guess is usually "water it", which is right about a third of the time and actively harmful the rest.
Here is how to tell them apart in under a minute.
First, feel the soil
This one test splits the field in half. Push a finger two knuckles in.
Dry. Light pot, mix pulling away from the sides, dust on your finger. The plant is thirsty. Water it thoroughly until it runs out the bottom, and check again in two hours. A genuinely thirsty plant is visibly back up within three or four hours. Some, like peace lilies, recover in under an hour and it is faintly ridiculous to watch.
Wet. Heavy pot, cold dark mix clinging to your finger, and it has been more than three days since you watered. The roots are in trouble. Do not water. Go to the next section.
If it is wet and still drooping
A plant sitting in wet soil that is wilting is telling you its roots can no longer absorb. That is root rot, or it is on its way there.
Slide the plant out of the pot and look. Healthy roots are firm and pale, white or tan, and they hold their shape. Rotten roots are brown or black, soft, and slide off in your fingers leaving a string behind. There is often a sour smell.
If most of the root ball is still firm, trim the mush off with clean scissors, repot in fresh dry mix in a pot with a drainage hole, and leave it dry and bright for a week. If nearly all of it is soft, the plant is very unlikely to come back.
If it is dry but watering did not fix it
Two possibilities.
The water ran straight through. Mix that has dried out completely turns hydrophobic and water channels down the gap at the side of the pot without wetting anything. The pot feels watered and the root ball is still bone dry. Fix it by standing the pot in a few inches of water for twenty minutes and letting it drink from below.
It is root bound. Roots circling the inside of the pot with almost no mix left cannot hold enough water to last a day. If roots are coming out the drainage hole, that is your answer. Repot one size up.
The ones that are not about water at all
Cold. Below roughly 12C most tropical houseplants droop, and the damage happens overnight on a windowsill behind a closed curtain. If the droop appeared after a cold night and the soil is neither wet nor dry, move it away from the glass.
A draught. An open window, an air conditioning vent, a door to the outside. Constant moving air pulls moisture from the leaves faster than the roots can replace it.
It just got moved. Plants droop after a change in light or a repot and then recover on their own in a week. Do not treat this. Leave it alone.
What you would have seen three days earlier
Every one of these had a lead-up. Soil moisture had been climbing for four days, or falling off a cliff. The temperature had dropped overnight. The tank was empty and had been for a while.
A droop is the plant running out of options. The numbers move first, and they move slowly and boringly, which is exactly why nobody catches them by eye.
The FACEPLANT Pot is built around that gap. It reads soil moisture, the water left in its tank, air temperature and humidity, and shows the state on its face, so the question you are answering is "what changed on Tuesday" rather than "why is it sad today". The app keeps the history by day, week and month, which is where the pattern actually lives.
The short version
Feel the soil before you do anything. Dry means water. Wet means stop. And if you want to catch it before the leaf goes over, you have to be watching the thing that moved first.