Related: Cannot access group. How do I rejoin my group chat? Why can't I message in my group?
Signal servers do not know your membership in a group, only another member of the group can refresh this for your phone.
To be able to message in your group again, please directly message another member of that group and ask him/her to send a message in that group. If you are on Android and still do not see any updates, go to Android Settings > Apps > Signal > Tap on Clear cache then ask your contact to update the group again.