This has happened to my guild as well.
Checking with several other members, to all of them it appears as if they are not in a guild anymore.
Most people have lost their Guild tag, but I was told that for my character
it does not show the guild name but I do have the brackets showing []
For guilds that are wishing to progress and build themselves stronger, the game has been poor about it from opening.
For the first week we had many guild members that could not Represent… several that were joined, but to did not see the guild roster themselves (and therefore could not represent)… and now the whole guild is gone.