Plants droop because of a lack of turgor pressure. This pressure is mainly provided by the presence of water in the stems. This water is moved through the roots and stems via transpiration. So, now that a few basics are out of the way... what is causing the loss of turgor pressure:
Too much water: If your roots are water logged, they is not enough oxygen present to allow them to efficiently transpire. It is a great deal more complicated than that but lets just say your roots are "rotting" (in a way).
Too little water: this is a no brainer. The plant is transpiring, or trying to, faster than water is uptaken in the roots. The hotter and drier the condition, the more the plants need to transpire.
Too much fertilizer: Water is absorbed because the plant roots are "saltier" than the substrate. If you have allot of fertilizer in the soil, then there is no difference in salt content (or gradient) to draw water into the plant.
Disease: Rotting roots or diseased roots. There are a number of different things that can bugger your plants.
MOST beginners see wilting from over watering. But if it has been several days..and your container is completely dry (the bottom is dry). then you likely have some root or fert issues. Not knowing anything other than you have a "plant that is wilting". I would say do the following.
Take the plant out and look at the roots...are the bound up or dark or rotten? If so, come back to us with what you see. If there are white then put the plant back in the pot, and flush it out with water and B1 vitamins. use a volume of water 4x the size of the pot.
Keep temps under 85 and humidity above 50%.
