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It is possible that the plant food was applied incorrectly or at too high a rate and this burned the roots, which then killed the plants.

The other possibility is that marigolds love sun. If they do not get enough sun, they will slowly sicken and die.

They may be beyond saving, but if you want to try, the only thing to do is to just give them good care and keep them warm. They may regow the leaves they lost if the stem is still alive. Remove any dead growth.

I simply deadhead the spent blooms, allow them to dry and then place them in a cool, dry location until the next season. I also place them in a brown paper bag for storing.

It could be slugs. This article should help: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm

You can also try spraying the plants with neem oil to prevent other insect pests from munching on them. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

I would treat both with neem oil. It is very effective and safe. This article will help: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

It could still be slugs as sometimes it is hard to see any trail. Here is a link to an article that should help you out: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm

I have used Slug bait granules and the beer methods before with great success.