If there's no word or a status indicating what happened, then consider them completely gone. I've seen people completely drop off the radar and never come back more times than I can remember. If there's a mystery status, then put them on the sidelines or background but keep on going with it. When they come back after a long time, there's a low chance of them being into it anyways, since they're just a little burned out, like going into an RPG save months after putting it down.