There are usually three things my GM does when a character goes missing.

• If a character isn't important, continue the adventure and have the missing in a "bubble" so they're there.... but not actually doing anything.
• If the character is important and if it's allowed, you can ahve someone else play the role of the character while playing their own.
• If they are important and there's not enough players to work with, then reschedule the session.

On rare cases, have a person on standby willing to play on that day if one of the players go missing.

There are usually three things my GM does when a character goes missing.

• If a character isn't important, continue the adventure and have the missing in a "bubble" so they're there.... but not actually doing anything. (This method is often preferred as it allows players who go missing to still get experience and items despite not actually being there as well as not worrying about their character dying while they're away.)
• If the character is important and if it's allowed, you can have someone else play the role of the character while playing their own. (While the player gets experience and items, they're not in control. Should something happen to the character such as death, there's nothing they can do about it. Which is why it's not really recommended unless you know you can trust other players with the character.)
• If they are important and there's not enough players to work with, then reschedule the session.

On rare cases, have a person on standby willing to play on that day if one of the players go missing. (Not really reliable since you can't really be sure the person would be ready on the drop of a hat.)