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.