One screw just came out and won't screw

It seems like maybe the screw mount has failed on you where the screw usually seats to hold the back cover on. I have had luck using a small amount of hot glue on the underside of the screw applied with a toothpick, simply to plug that hole and avoid dust and other particulate having easier access to the inside of the case. Otherwise, if all your other screws are intact, it should not be much of an issue for you. This is intended to be a cheap and quick fix though, and if it concerns you a lot, other solutions may be required. In the case of an apple laptop… replace the chassis?