If you go to a Meetup and the Organizer doesn't show up, then the first thing you should try to do is contact the Organizer in order to find out what happened. Something may have happened to prevent the Organizer from showing up for the Meetup.
If you do find yourself a member of a Group that's gone inactive, you've got a couple of options.
If you're on the group's leadership team, you may be able rally other members to have a Meetup on your own. You may be able to start a new thread on the Group's message board, or send a message to the Group's mailing list. You'll see those options if they're available in the top navigation bar on the Meetup Group's home page.
In some cases, if you're not able to get in touch with the Organizer and you feel he or she isn't doing their best to organize the kind of Meetup Group you want to be a part of, your best bet might be to join a different Meetup Group.
Or if you're up for it, you can start your very own Meetup Group and invite others to join it. That way, you'll always know whether or not the Organizer is going to show up.