It happens to the best of us. One day you find a plant has collapsed. Here's a quick run down of the most common reasons for plant death:
Dry Soil No plant, not even a cactus, can survive without water. While many plants are able to subsist for long periods of time with infrequent watering, they will eventually begin to wilt and eventually die. Every plant's needs are different, and purchasing a moisture gauge can help you to water properly.
Overwatering This is probably the most common cause of plant death. Overwatering deprives the roots of water and causes them to rot. It's important not to confuse the symptoms of over and underwatering. Both cause plant death, but an underwatered plant will have brown, dry leaves while an overwatered plant's leaves are usually yellow and wilted, and the stems may show signs of rot. Again, a moisture gauge is helpful here.
Cold Nights Many plants love a sunny windowsill but many people don't realize that plants can easily succumb to frost on a cold night. Never leave plants between the window and closed curtains on a cold night. If the room is left unheated at night, remove them from the window.
Direct Sunlight While some plants, especially cacti and succulents, thrive in strong sunlight, many others will suffer. Delicate leaves are easily burnt, especially if water drops are left on them, and soil quickly bakes, killing roots. It's important to protect plants from the strong midday sun of the summer months. This is easily done by moving a few feet away or even by placing a curtain between the plant and the window. Never put a plant that has been sprayed with any kind of fertilizer, fungicide, or insecticide in direct sunlight as the wet leaves will burn.
Dry Air Most houseplants are from tropical regions and as such appreciate humidity. Hot dry air, especially that given off by indoor heating in the winter, is a plant's worst enemy. You can combat this by misting plants, setting them on trays of moist pebbles, or even by simply grouping your plants together, as they will naturally increase the humidity of the air around them through transpiration. A humidifier is also helpful and may be appreciated by the humans in the house as well!
Drafts Houseplants hate drafts. Some, like Chinese Evergreens and Diffenbachias, will immediately show their displeasure with yellowing leaves. Never place plants in a direct line between a door and a window. If your plants are on your windowsills, make sure there are no cracks letting air in from the outside.
Poor Light All plants need light to grow, even those marked as "Low Light". There is no plant that will thrive in a dark corner or hallway. If you want to keep plants in places such as this, you'll have to rotate them regularly to well lit spot for at least a week at a time and then back to where you want them. It may be more convenient to buy artificial plants for such areas instead.
Hopefully you'll know be able to figure out what killed your plant and be prepared to prevent it from happening again!