They lasted a whole year??? Mine would only last just a season... They are only seasonal flowering plants. So just pick their flowers when they dry up. Then split open their base, and inside you'll find some small toothpick like bristle. Mostly white, but one end of them will be dark - That whole thing is their seed. So what we do is collect them, to replant next growing season. Because as far as I know, marigolds aren't perennial plants... We in my locality plant then in late winter, just before the arrival of spring. they have a magical quality of working like an insect repellent. So they are ideal for planting around your vegetable patch. Over here we get them in 3 colors, white, yellow, and orange.
There are over 18 types of Marigold and more than 50 Species of them. Ranging in size from dwarf to 3 foot tall giants. Some are annuals meaning they grow - bloom and die all in one year and do not come back the next year, some are perennials...meaning they come back from the roots every year. And pretty much each variety looks a bit different from other species.
What we mean is, how did they look when they were in full bloom? If you have a picture of that stage of their life - Then we might be able to identify the reason why your plants are like this now.
