It's tricky to diagnose a plant problem from a photo or verbal description. To be safe, I'd recommend taking it to a local Master Gardener clinic to have it properly examined. See www.kingcountyMG.org for a clinic near you.
Now, let's think about some possible causes. More often then not, plant problems stem from care, which then can lead to pests or disease.
1. Is it just one leaf? Or are the other leaves starting to split and brown on the end? Be careful when watering not to let the leaves get drenched. Don’t water down the middle--orchids don’t like water sitting on their leaves.
2. Check the leaves for signs of pests. Orchids should be monitored for aphids, mealybugs, mites, and scale. These can usually be removed with a soft cloth and soapy water. If you choose to use a commercial pest control product, be sure to follow all label instructions.
If you suspect you have a pest, make sure you isolate the plant from other houseplants until you have the problem under control.
3. Are you over or under watering? Water every 7-10 days, preferably in the morning, and let the water completely drain out of the bottom of the pot into the sink. Don’t let your plants sit in water. This often causes just the right condition to encourage pests or disease. Phalaenopsis do not like to be dry to the point of wilting. They should be watered thoroughly and then not again until the media is nearly, but not completely, dry. For added humidity, especially when your furnace is running, set orchids on a tray with some pebbles and a little water.
4. Are you fertilizing? I don’t fertilize when in bloom. After the flowers are gone, add a pinch of orchid fertilizer, available in many good nurseries, to about a cup of water. I withhold fertilizer about once a month. Over fertilizing can lead to leaf problems.
5. Will my plant bloom again? Well, as they say, “Location, location!” One of the most common mistakes made is placing orchids in direct sun, which will scorch leaves, or placing orchids in dark rooms, which will lead to no blooms. Phalaenopsis like in-direct light.
6.And finally, there may not be anything wrong with your plant. Sometimes you just have to prune off a leaf. :-)