Did you just buy it? Many aquarium plants are raised out of the water, called emersed. This is done because there are benefits for the vender. The plant grows more quickly with co2
from the environment, there is no algae to worry about, and they can use natural light (Florida is one of the places they grow these plants). The can also pack more plants into a smaller area. Okay, so you bought the plant. Was it underwater? Sometimes you can get a plant from a fish shop that is submerged. When a plant is submersed, (especially a rosette plant-like amazon sword), it loses its above-water leaves and transitions into a below-water plant. So what you are seeing is the transition from a terrestrial plant into an aquatic plant. In many places where these plants grow, they have rainy cycles, and then dry cycles. A lot of these plants have the ability to "die" and become the above or below water kind. This is what you are experiencing but you don't know it. So you need to just take the OLD dead leaves off and you will eventually see small leaves coming from the middle. They will grow, slowly, but then in a few months, you will have a brand new plant. This should be a picture of one of the swords in my tank. I got it as a tiny plant, and it is the size of a cabbage now. It took about a year to grow this big, sorry to say. Yeah, you need to be patient. Sorry it is sideways. I am just figuring out pictures here.