What Albert & Ricky said.[br]
Additionally, make sure you are using a manufacturer recommended charger and a known-working cable for your phone. Also, make sure to plug the charger directly into the wall, not into a power strip or extension cord, as those can sometimes have trouble providing continuous electrical flow to your phone.[br]
If your phone is already on but not charging, doing a Restart can fix the issue. I’ve seen older Samsung phones charge normally after a standard soft restart.