When you set your preferences when first signing up with your group on BigTent, you may have chosen to hide one or more forums by setting your email options for those forums to "None, hidden." This means you will not be able to see these forums online. You can change your settings for forums at any time by going to settings > email options > forums.
If you are unable to view ANY content for your group (forums, news, events, etc.) it is possible that your membership has expired. If you think this is the case, contact one of your group's administrators so that they can invite you to rejoin the group.