Orchid having problems - "stem" turned yellow, now spotted leaf?

Reading your posted question I think you are possible talking about a Phalaenopsis Orchid. If that's the type of orchid you have I would say the flowering time has finished and all the orchid flowers on the "brown stem" as you call it have finished their life and fallen off the stem just leaving the empty brown stem behind. The large green elongated wide leaf sometimes also at the end of the flowering period can begin to die off and become a dry wrinkled brown leaf. There is nothing to worry about if that's what's happening to your plant. Cut the old dry stem off and when the dry leaf has fully dried out you can give it a gentle tug and it will possibly come away from the plant. New growth will eventually grow from the centre part of the plant and is normally a new leaf and all going well you will notice a new flower spike also coming from the centre of the plant possibly early next year. These types of orchids are basically an indoors plant and the leaves etc can become sun burnt if you place them outside. Ideally the plant should possibly be re potted now with new Orchid potting mix as it appears the plant is outgrowing its' pot. They are pretty hardy, just remove the whole plant gently from its' pot, clean up any damaged or rotted roots and then put it in the next up sized pot that what you have removed it from. Orchids need to be cramped in the pot as that makes them flower again in the new season. If you put them in a much larger pot it probably won't flower for several years. They flourish indoors as long as they can get in-direct sunlight but also do well on large window sills facing the outside but not against the gl as winter ice and snow will burn the leaves in winter. Put the plant on a saucer with about 3/8" of sand and just keep it moist not wet or it will rot. Maybe a small diluted dose of Orchid food later, just before it begins to grow again.