Follow this Apple T/N [https://support.apple.com/en-us/HT203207|Apple Portables: Troubleshooting MagSafe adapters].
You may need a new charger do make sure you get a real Apple unit [https://beetstech.com/blog/oem-magsafe-chargers-vs-counterfeits-teardown-for-truth|OEM MagSafe Chargers vs Cheap Imposters: Teardown for Truth]
Within your system is the DC-In board, in this series its part of the I/O board if the contacts look bad I would recommend replacing it [product|IF108-049] and here’s the guide to put it in [guide|38386]