Please take a look at the cropped screenshots of two forums below. Both are from the same course. One is a March 23-27 forum and the other is a forum for March 30 - April 3. Notice the icon that looks like a small person beside the Edit option. This icon changes the settings for a Moodle feature called "groups." However, if you aren't using Moodle's "groups" feature, this button doesn't do anything useful in your course, but it's still ridiculously easy to click it by mistake and toggle the settings for groups. Changing the groups options when you aren't using groups will lock everyone out of the forum because no one is in a group. It is one of many annoying idiosyncrasies within Moodle. The good news is that it is easy to turn it back off by simply clicking the icon twice to bring it back to the default. If you're not using groups, you want the icon to look like the icon that's shown beside the March 23-27 forum.