My friend from elementary school has barely been replying to my messages and doesn't reply at all the most I get is one sentence?

They're either isolating themselves and going through stuff and doesn't feel like socializing right now or yall aren't as close as you think. Either way just let them be, at best reach out with a simple text asking if theyre good or say youre just checking in on them. keep it light and simple. make sure you let them know youre there for them, leave it at that. Dont allow yourself to get affected by it. They're clearly going through something, just focus on you and your well-being.