Moth orchids are in genus Phalaenopsis, abbreviated Phal., and are mostly hybrids. They need warm termperatures, shade with no direct sun ever on the leaves, and plenty of air at the roots. Most of the ones sold in markets come in thin transparent plastic pots with sphagnum moss. If the moss is watered to the point of being wet too often, and the roots stay wet, they can rot.
If your plants are potted as I mentioned above, slide them out of the thin plastic pot. Do the roots look green, or are they black? If they're green they are still alive. Leave the plant out of the plastic pot someplace with good air circulation, like under a fan, so it can dry out.
If they're black, pull off all the moss. Are there any green roots? If so, let the plant dry out for a day and repot. Read the first few pages of this thread:
The Phal abuse ends here.
If the roots are black, cut off as much as are black and slimy. Leave the parts that aren't black. Are there any roots left?