What will happen to plants that are kept in the dark?

It wont get the proper sunlight it needs. Sunlight gives the plant energy.  

Light is the energy source that allows plants to convert carbon dioxide and water into complex carbohydrates such as glucose. Some plants need direct sunlight to thrive, but others do better with less light. Plants that prefer shade, such as ferns, violets and begonias, are popular plants for apartment dwellers because they do well in indirect light, but no plant can survive for long without any light at all.
While temporarily placing a house plant in a dark room may not cause it any immediate harm, all plants eventually die without sufficient light. Plants need light for photosynthesis, which is the process by which plants make food.
Hope this helps :)