That depends on what kind of orchid and which leaf. On a Phaleanopsis, if it is the lowest leaf, it could be normal. If it is more than one leaf, it might be root loss.

The plant is not getting enough water. That doesn't mean necessarily that it's been underwatered, it is quite possible (and more likely) that it has been overwatered and its roots have rotted away. The only way to tell for sure is to look at the roots, if they're dry, wrinkly and shriveled then it's probably due to underwatering - a good soak in a bucket of water will help it regain some if its lost moisture. If the roots are brown and mushy, then the plant has been overwatered. In this case the best thing to do is to reduce watering frequency and when you do water, water it with SuperThrive or another rooting hormone. If the medium it is growing in is broken down (if any), it may be time for a repot as well.

To give you any further information, we'll need some more info from you:
- What kind of orchid is it? Phal, vanda, dendrobium, etc?
- Is it growing in a basket or a pot? If potted, what is the growing medium?
- Where is the plant growing? indoors (which side of the house and how far from the glass) or outside?
- What kind of light does it get? Direct, indirect? How many hours?
- How often do you water?

Any any other pertinent information you can think of.
Best of luck,
~Jem