In case you need a refresher, Ant-Man and the Wasp’s mid-credits scene picked back up with Scott, Hope, Hank Pym and Janet van Dyne at the exact time that the battle in Wakanda was unfolding in Avengers: Infinity War.
Scott entered the Quantum Realm to harvest some quantum energy that would help Ava Starr’s condition, and he was only supposed to stay there for a few minutes.
Unfortunately for him, soon after he traveled to that other reality, Thanos carried out his genocide, and Hope, Hank and Janet were among the unlucky ones who disintegrated.
Scott is still intact, but now he’s stranded in the Quantum Realm, and it remains to be seen how he’ll escape, although many have speculated it will require him to go through one of those time vortexes that Janet told him to stay away from.