Anyone good at gardening who can help me with my plant?

I planted some cosmos in a pot. I don’t think I should have, but I wanted to put something in a pot. I wanted to grow something from seeds. So I tried it. It was really healthy and growing great, but it’s now turning limp and brown. I circled some brown stalks and this was taken days ago. It’s now more brown a lot more wilted. Is there anything I can do to save it? My stepdad suggested the roots are strangling one another because they’re too close because the pot is too small. I have no idea if that’s a real thing or not lol so I figured I’d ask. Should I try to repot in multiple different pots?