Your plant looks like Tagetes patula, common name French marigold, and I'm sorry to say that they only last a year, or one growing season, because they are annuals. That means they grow from seed into plants, start flowering, continue flowering for some months, and then just die. In colder regions, they die as soon as the first frost arrives, but even in zones without frost, they will die anyway. There are one or two perennial marigolds - Tagetes lucida (common name Mexican/Spanish tarragon) and Tagetes palmeri (Mountain marigold) if you want a plant that lives longer. Tagetes lucida is similar in appearance, but usually has single flowers. https://homeguides.sfgate.com/marigolds-perennials-annuals-63218.html