Pseudobulbs that should be plump are now shriveled and have deep furrows running lengthwise.
Leaves like the Cattleya
leaves are start to curl, soften, and develop creases.
Leaves like the type Miltonia
and the Oncidium
have never expand and look pleated like an accordion.
Blackened areas - especially around the bottom of the plant.
Older leaves turn yellow and drop off.
The media smells sour. If possible, gently remove the orchid from its pot enough to examine the roots. If the roots are dark and mushy, then the plant is most likely suffering from over-watering.
What should I do?
By the time you notice that there is something wrong, the media has already started to decay and the plant has begun to loose its roots. You need to repot immediately and cut off all dead and decaying roots.
Use a coarser grade of potting mix
since this will allow more rapid drying out. Since the plant probably has very few roots, support it with a stake.
If the plant has no (or very few) living roots left, you can try placing the orchid in a plastic bag. Loosely tie the bag closed. The bag will help the orchid from loosing more moisture by reducing evaporation. Place the bag in an area that only receives indirect light until you see new roots growing.
Do not place the bagged orchid in the Sun!
When you see the new roots, take the orchid out of the bag and place it on a humidity tray. If your plant has black rot, treat it with a fungicide before repotting. Be careful when you start to water again.