I got a cute little marigold plant in full bloom about a month ago. Ever since I got it, it has been going downhill. I keep it in sunlight most of the day, and water it when the soil is dry up top. It's leaves are shriveling up and the flower is dull and limp. The leaves are dry to the touch if that is important. I'll add a couple pictures of where I keep the plant, the soil I use for it, and what the Marigolds look like too.
I'm afraid the plant is dead and desiccated. In Florida it's really easy for them to get baked to death.
The good news is that the experience is very useful (for the gardener) and you have lots of time to attempt again. I would try growing them from seed because they're extremely easy. At the Dollar Tree you can get 3 or 4 packets for $1 and potentially grown dozens of plants which could be blooming in 5-7 weeks from the time you plant them.
I have planted in containers sitting on the ground on the west side of the house and a couple planted in the ground on that side as well; they are all still going strong. They get a lot of sun every day but only receive water twice a week when the irrigation system is in operation.
At our old house a few years ago, I had Aztec Marigold (Tagetes erecta) growing in a few spots in the garden and they would readily reseed each year providing new plants.
~ I'm an old gal who still loves playing in the dirt!
~ Playing in the dirt is my therapy ... and I'm in therapy a lot!