It has not been closed or archived. It is not very active and the last post before yours was 2 weeks ago. That tells me you have an age filter on and set for 2 weeks. I'll have to find it, but there is a setting somewhere that controls the age of the posts you can see.

Edit: Found it. Go to your control panel (top right part of the page, called Settings). Under General Settings, scroll down until you see "Default Thread Age Cut Off:" Change it to whatever works for you.