Have you tried these steps?
- Press ‘J’ on your keyboard to open up your guild window. This is the window which shows all of the guild information.
- Click the ‘Roster’ tab along the bottom of the window.
- Find your character’s name and right click it. A menu will drop down from your name with several selections.
- Select ‘Leave Guild’.
Generally you can quit the guild from your name in the guild tab, or you can do so through /gquit. If that doesn’t work, you could try a full UI reset.