Roll20 has an issue where that message appears if you have been removed from a game by a GM. Staff have said this is not a bug, but working as intended. They presume if a GM removes a player it is for cause and that they want to block them on Roll20. Unfortunately, most folks don't know this and GMs frequently remove players from games all the time for no negative cause. In your case, once the oneshot was over, the GM probably removed all of the players to free up the game for another set of players. It is also possible that a GM deleting a game with players still attached does the same thing but I have not seen any confirmation of this. I would recommend sending the GM in question a private message and ask him to add you directly to the game but this may or may not work. I think I've seen a report where the GM was unable to add a previously dropped player. In that case, the GM would probably need to open a support ticket with Roll20 to get it done.
