There may be a couple reasons why your guild tag is not being displayed:
- Press 'Esc' and select 'Settings.' Then under 'Interface' select 'Names' to make sure you have 'Show Your Name' selected
- Verify that you're still in a guild. Press 'G' to bring up your guild window; if you are unable to bring up that window then you are no longer in a guild
If you have 'Show Your Name' selected and you are still currently in a guild, ask an officer of the guild to remove and re-add you to resolve the issue.