On the last message where the moderator said they closed the thread, or maybe any of the posts in the thread, you can click on the little exclamation point in a triangle and a "Report Post" message will appear. You can then ask why it was closed and a moderator should get back to you, I think. You could also PM the moderators directly.

What I've seen recently is that threads were closed when they got off-topic and into repetitive or unpleasant or personal disagreements. I think that's considered against community guidelines but a moderator could tell you for sure.