I had same issue here, no guild chat, MOTD, ‘J’ tab not showing guild, etc. Affected every character I had in the guild (10+), and quitting/re-inviting them all wasn’t really an option.
But, after raid tonight I had an officer invite another character on my account that wasn’t in a guild, then logged off and back onto my affected main and it was all working again, all my other characters are working again too.
I can’t say for sure the inviting of the character fixed it, could just have been fixed while in raid and needed a re-log to go through, but worth a try for anyone affected who can get another character invited.
Obviously this won’t help anyone in a dead-guild, bank-alt guild, solo guild, etc.