If you find yourself unable to join your friends in the room as usual and you don't have mismatched game versions, it most likely means that your account has been removed from the main server for unfair and illegitimate progress changes, 3rd party software use, account modifications or hacking.
Please check our Community Code and Terms and Conditions for more information.
If you find that the removal was unwarranted, please check: I think my account was removed from the main servers unfairly.