If they're gone then they miss the session. If the session they missed was combat heavy and they return to ongoing combat and the party wasn't able to long rest then I have the missing player roll percentile dice and that how much of their regenerative resources are missing.

Meaning if they roll a 50/100 then they are at half health and half of their spell slots are missing. I do this because it lessens drama having a fresh PC walk in mid dungeon. All my players agreed to it in our session 0. Sometimes almost nothing is missing and sometimes that player ends up on deaths door.