A lack of water is one of the most common causes of wilting, especially among common house or garden plants. The cells in the leaves and stems become too weak to support the weight without water and the plant begins to droop. Try reviving a dry plant by sitting the pot in a tub of water or your kitchen sink and allowing moisture to soak up into the soil until the surface is wet. This ensures even moisture throughout all of the root system. For plants in the ground, water thoroughly around the entire plant.
If your plant didn't recover from wilting after a good watering and has yellowing leaves, root fungus may be the culprit. Damp soil encourages fungus to grow rapidly, and many fungi found in soil attack the roots of the plant. Most plants don't recover from this attack because by the time the leaves wilt the infection is widespread. The only option involves digging out the roots, cutting off any infected and soft pieces and re-potting the plant in pasteurized soil.
Over-watering causes wilt just as badly as under-watering. When roots stay wet because you constantly drench the soil, rot and mold develop that prevent the roots from absorbing water correctly. There's little to do for an over-watered wilted plant except to let it dry out as much as possible. If the soil dries, the dead and rotten parts of the roots will decompose and new growth may develop. Catching your over-watering habit before the plant begins to wilt greatly increases its chances of recovering.
Fertilizers help your favorite house and garden plants grow and produce large blossoms or fruit. However, too much of a good thing will lead to wilting leaves and drooping stems. Follow the fertilizing schedule recommended by the manufacturer to prevent build-up in the soil. If your houseplant wilts following a heavy dose of fertilizer, saturate the soil with water as soon as possible to flush as much of it out as possible before the roots absorb it. Re-potting may also help if you used a liquid fertilizer and act quickly.