If a player uses a key to open the hatch or there is only one player left and then the killer closes the hatch the endgame timer starts. But during the endgame timer no generators can be repaired. So if the remaining survivors don't have a key are they just dead?
I have actually had a game where this happened to me.
If a hatch is closed, that was opened with a key, endgame will start like it would usually do when you close the hatch. The generators will be blocked and the remaining survivors will be able to open exit gates. It's exactly what happens, when you close the hatch on the last survivor and not all the generators have been repaired, just with more players remaining in the game.
If after starting the endgame this way you manage to kill all survivors but one or they manage to escape, the hatch will open again, like it would under normal conditions. Closing it again will not grant you and more bloodpoints but it will hinder the last person from escaping through it.