Can a question be closed after a correct answer has been selected? I was answering a question then I noticed a reply had a check mark, did I miss that or can it load after the page initially loads? Then when I went to submit my answer I was unable to because the question had been closed.
Yes. Questions can be closed at any time. The Person asking the question decides whose answer to accept. The community decides whether the question belongs on the site.
And yes - if an existing answer is accepted while you're writing a new one, the checkmark will appear on it immediately. That doesn't mean you can't or shouldn't still answer though, if you think you can provide a better answer than the one that's accepted.