How can I save my injured marigold?

Marigolds are generally annual plants. The stem is turning red because it is getting old. The bottom leaves are dropping off from the drought stress; you will not be able to save them. The plant has probably out grown its pot and is root bound. You should repot the plant in a larger pot with fresh potting mix. Do not bury the bottom half of the plant, though. This will just promote rot. If you're worried about the plant falling over, just stake it up. Marigolds love full sun. The plant is most likely not getting enough sun, which is why it has not bloomed properly.