What Causes Yellow Leaves on Plants
There could be many reasons why the leaves on your plants are turning yellow. One could be a lack of water. Also insects can do damage and turn leaves yellow, your plant may need re-potted or a lack of light. You can find more information here: http://www.gardeningknowhow.com/problems/plant-leaves-turn-yellow.htm
Most of the time its caused by over watering. But it can also be a sign of poor potting soil. Also you might want to move the plant to get more direct sunlight.
Can my orchid be saved once the leaves have turned yellow and fallen off?
If it is indoor plants then maybe the plants didn't receive the enough light they need. You can use LED grow lights that you purchase online at http://goo.gl/jdxMsj.