You have a Phalaenopsis orchid, and if it did not come with a cultivar tag, then it's a no i.d. (or NOID). Blooms usually last about 6-8 weeks, so it might have been time for them to drop and the stem to dry up. Sometimes the stems will stay green and push out another branch, but sometimes they dry up. Since you have another spike still in bloom, I would cut the other stem all the way to the base of the plant (as close as you can get it without cutting the plant).
Also, I don't think light will really affect the stems. An indication of too much light will be reflected in the leaf color. If you're pushing the boundaries of light intensity, sometimes the leaves will become reddish tinged or pale greenish yellow. Most of my phals have a apple green leaves with a slight reddish tinge, but I like to push the limits on light to make sure they're getting enough. Dark green leaves usually indicate orchid needs more light.
As for watering, be sure to water with lukewarm water.
Once the other flowers drop, I would recommend repotting to fresh medium. This will allow you to get a good look at the roots to make sure it has a healthy root system. What is it currently potted in? If it's in moss, you will want to water when the moss dries (maybe every 10 days to 2 weeks). If it's in bark, maybe once a week. Since I don't know your growing conditions, it's hard to be more specific. I would advise to err on the cautious side with watering to avoid overwatering.
Here is a link to Phalaenopsis care. A sunny east windowsill is a great place for phals.
AOS | Phalaenopsis
Good luck and welcome to Orchid Board!