Edit: I can access the room again.
The room was auto-deleted for inactivity. Inactive chatrooms are frozen and if they do not contain much content they are even deleted (where as usual on SE 'deleted' just means soft-deleted).
A moderator can undelete the room with one click. (Since it is chat a moderator on any SE site can do this, thus I did it.) It should then stay in that state for some days, but will be deleted again if there is no further activity.
As a general procedure I would propose to flag the post to which the chat is attached for moderator attention and ask to undelete the chat. If it is not attached to a particular post I would ask in the Math Mod's office.