The plants leaves and/or stems are brown, dried out, and/or crispy.
There are many things that could cause your plant’s leaves to dry, turn brown, and die. Lack of water is the most common culprit in container gardening. If you have forgotten to water the plant and the reservoir and soil have dried out, the plant will first wilt down and eventually die. This would happen over a period of days and would affect all the plants in the planter.
If there are only a few leaves that have turned brown or died this is probably just part of the natural growth habit of the plant. Plants, and their constituent parts, have lifespans; new growth replaces old growth and old growth dies away. Normally, new growth replaces old growth so quickly that you may not notice the death of the old growth. You may consider trimming off dead or old growth to encourage new growth to replace it and for aesthetic purposes.
If, however, you have kept the plant generally healthy by regular watering and it suddenly turns brown it is probably due to one of two common factors: chemical burns and/or advanced disease/insect problems. Browning caused by chemicals and fertilizers are discussed below. For more information on infestation and infection problems see pests and diseases.
Is it possible that the plant may have been sprayed by a weed killer or other chemical? (For example, you have recently had exterminators or lawn-care professionals using sprays in your yard or your neighbor’s yard.)
If so, the plant has been injured or killed by an herbicide or burned by a strong insecticide or pesticide.
Prune away the dead, if there are any living plants, prune carefully to encourage new growth. If there is only one bare spot, carefully prune the plants around it to encourage bushier growth to cover the spot. If many, or all, of the plants are dead or severely damaged purchase new plants to replace them.
Do you fertilize EVERY time you water?
If so, the plant is probably getting too much fertilizer. Make sure you water at least once between fertilizing times. If the plant is dead or severely damaged purchase new plants to replace it. If the plant has some damage, prune away the dead parts to encourage new growth from the center.