It has been some time since I’ve gotten onto the forums and was shocked to see a forum that was “closed.” I’m confused and hoping someone can help. Why would a forum be closed and who does this? Is there a way to reopen them?
The forum features have a forum (confusing, read it twice. :)), so that only the CD bot can start threads. The forums from past years are also closed, for obvious reasons. 
The moderators have closed many threads that get out of hand or become a “flame war”. Most threads can be reopened by a moderator. Also any thread that has been inactive i think for a year gets closed automatically and can be reopened. If there is a specific thread your wondering about post a link to it and a better answer can be given.
Do you mean ‘forum’ or ‘thread’?
A forum has a lock on it, because you are not allowed to start threads in there.
A thread could be locked for one of few reasons. 1) its old. 2) the topic has been beaten to death. 3) abuse. 4) duplicate thread. 5) something else.
If you have an example, PM me and I’ll let you know what the reasoning was behind closing it.