A cell phone may not charge if the charger is not working, the charging port is dirty or broken, or the battery is worn out. Occasionally, the battery may need to be reset, which is completed by turning the phone off and on again.
To determine the reason why a phone fails to charge, first try charging another phone, if available, with the same charger. If the charger is working, check to see if there is any debris or dust in the charging port on the phone. Finally, try moving the charger around gently to see if a proper connection can be made. If none of these methods work, the battery may need to be reset or replaced.