Wilting means that the plant is not able to replace the water lost by evaporation and transpiration. To determine the cause, you must examine the roots of the plant.
Over-watering will cause the roots to die from lack of air. Look for wet, saturated soil caused by lack of drainage from the pot. Provide proper drainage and reduce watering.
A pot-bound plant has filled its pot with roots which can often be seen growing out of the bottom of the pot. There isn’t enough soil to hold the water for all those roots, so the plant wilts. Correct by re-potting in a larger pot.
Fertilizer and minerals from hard water will accumulate in soil and cause root damage and wilting. The best solution is to re-pot in fresh soil.
Fungal root rots will eventually cause a wilted top. Almost all healthy plant roots are pale in color and firm. Dead roots are brown to black and may be soft. Certain fungi cause root rots which are usually aggravated by too much water. If only a small amount of the roots have been affected, drenching the soil with fungicides may cure the disease. However, most indoor gardeners will find that these chemicals probably cost as much as a new plant. Good results are usually obtained by cutting dead roots and re-potting in clean soil. If most of the roots are dead, the plant should be discarded.
Sometimes infection by root knot nematodes will result in wilting. Look for lumps and bumps on the root system. Infected plants and soil should be destroyed. If in doubt, submit a sample to your extension agent for confirmation.