The mid-credits scene of Ant-Man and the Wasp saw Scott Lang stranded in the Quantum Realm. He headed into the Quantum Realm to obtain more of the healing particles for Ghost, but this meant he was inside the Quantum Realm.
Everyone disappeared. No one was there to bring Scott back. Time travel is a factor in the film, and now that Ant-Man and Wasp have revealed that time vortexes exist inside the Quantum Realm, and they could potentially give Scott Lang an escape route.