If your iPhone shows 0% battery even after plugging it in after a battery replacement, it likely indicates a problem with the connection between the new battery and the phone's charging circuitry, potentially due to a faulty battery, damaged charging port, or a loose connection within the phone itself; check the charging port for debris, try a different charging cable, and if the issue persists, consider taking your phone to an Apple authorized repair center to have the connection inspected and potentially re-soldered.