There are many types of Orchids with various care needs. Knowing which type of Orchid you have will help determine its care. But generally - Overwatering is a common cause for dead orchids.
How often you should water a plant depends on:
• How much water it uses (likes it Dry or Wet)
• Function of Humidity, Light, Air Movement, Growing Media-Bark, Moss or Soil.
• When to water most orchids= Just before it goes dry. It can vary from every few days to every couple of weeks.
• Orchid media decomposes over time, especially bark. When this happens, the bark loses the fast-draining properties that many orchids prefer. That's why it's necessary to repot in new bark every year or two. Use an “orchid” fertilizer or G&B All Purpose or Bloom liquid fertilizers (follow directions on bottle) at half strength each time you water or monthly.
For more detailed information on Orchid Care, refer to the following links:
and the American Orchid Society: www.AOS.org