The GM or other players usually have copies of the missing PC's sheet, so it's not a problem: someone else usually plays them, so they get full XP, treasure, etc.
Sometimes that's not possible, or the person has not left their sheet. In that case, sometimes something befalls them (illness, spends time in prison, something like that) so the absense can be explained. Usually they don't get XP. The current game we're doing, a 'modern magic' campaign using a mix of the CoC d20 rules and the Shadow Chaser gamelet from Dungeon, one person has missed two sessions in a row (work took him out of state) and he's a level behind the rest of us. No sheet, and the events that happened were... well, there was no way for us to judge what he would have done even if we'd had the sheet. We don't know his character that well, yet, for any of us to play him with anything approaching accuracy.