Most live problems have a quick fix. Start with the one that matches what you see.
A player cannot join
Check, in this order: the event is started (players cannot join a Not started event); they typed the access code correctly; the game is not at its player limit; and they chose a player name. Scanning the QR code again is the fastest way to rule out a typing mistake.
The big screen (HUD) is blank or frozen
Refresh the HUD window. If that does not help, close it and choose Open HUD again from the Host Console. If it is a casting problem, try an HDMI cable. See the HUD.
A player's phone lost its connection
Ask them to reload the page, or scan the QR code again. If they are not in your player list afterwards, have them join again with the access code.
I closed the Host Console by accident
Closing it does not end the game. Open the event from My Game Events and choose Open host console. See how to open the console.
Players are seeing a player limit message
Your plan's limit has been reached. See player limits for the messages and how to add room.