Hi Steph,
Welcome to Orchid Board. Your plant is probably suffering from too much moisture. Sphagnum moss has a reputation for staying wet and with no air holes in the pot I suspect your roots are rotting. These type of Dendrobiums need to be potted in a well draining medium and specially designed orchid pots help as well. Orchid pots have more slits and holes than the average pot.
I'd remove the plant from the pot and check it's roots. In your fourth photo I see an example of a rotted root. It's the brown thing with a wire looking thing coming out the middle. Carefully strip all the sphagnum moss from the roots and check for more roots that look like the one in the photo. Use very clean scissors or clippers and remove all roots that look like that. Feel all the roots that are brown. If they're mushy then remove them too. You can treat all cuts with a little dab of cinnamon from your spice cabinet. It helps to fight bacterial infections. It shouldn't hurt your plant if you let it air dry for a day or two. It may even help. While it's air drying you can find a proper container to plant it in. Be conservative with the size of the container. These plants grow better when they're a little restricted. Since you're a beginner I would suggest you buy some premixed orchid plant mix. Lowes, Home Depot, and Walmart carry it. Read the package and make sure the type you buy is recommended for Dendrobiums. Soak the medium several hours before you repot your baby. A few pebbles in the bottom of the pot will aid in drainage. You may have to anchor your plant inside the pot so it doesn't fall out. This can be done with wire or specially designed orchid clips (also available at the above listed stores).
Don't be tempted to remove the leaves or the canes their growing on. The leaves will most likely wither and fall off naturally. The remaining bare canes will provide energy for the plant while it recovers.
Don't water for a week or so. Remember the new medium was soaked before potting, so it will stay wet for while. When you resume watering do so sparingly by pouring water through the top. You should only have to water every 4 to 7 days. Don't let the pot stand in water. Just let it dry out naturally. Keep in mind that these plants like warmth and humidity so you may need to place a tray of wet pebbles beneath the pot.
It might also be possible that your plant has received too much sunlight. Phal type Dens. like light but not too much and not too strong. A dappled eastern exposure works well for me.
In a few weeks, if all goes well, you should begin to see new growth (canes) popping up near the base of your plant. Newly developing canes will provide your plant with fresh green and white roots. Just be careful to not overwater them or they will promptly rot.
You can begin using a weak solution of fertilizer in about 2 weeks. Better-gro is also sold at the above listed stores. This brand has worked fine for me when I mix it half strength. Avoid the use the bloom buster fertilizers at this time because your plant needs to recover before it blooms.
Your Dendrobium looks a little sad now, but she should make a full recovery with a little knowledge and TLC. Best of luck.