The simple answer: Lack of water or dehydration. The next step is to determine why the plant is not getting sufficient water. First, look to the roots. If the roots appear healthy, white or green and plump, and medium is in good shape, suspect underwatering, especially if the roots are white and the pot is very light. If, on the other hand, the roots are in poor condition, suspect root loss. If the plant has no roots, it cannot take up any water, no matter how much you give it. In this case, the cause may be root loss owing to overwatering or medium deterioration, or a recently repotted and poorly established plant. The immediate solution is to raise humidity in the plants’ vicinity to reduce stress on whatever roots there may be, and then deal with whether to repot or to simply wait until the plant establishes in the fresh medium.