Not to pile on but I, too, am having the same experience of not being able to access two of the groups I myself created for my classes. I've pointed my kids to resources I've posted to this class group account a week or so ago and now, as luck would have it, no one including myself can get to it!!
Like johnos says......Help!

Dear Lei Zhou,
Thank You Thank You Thank You Thank You
Your quick response & solution are most appreciated!!!
Harold J.