The charger, the cable and the outlet
Try another outlet, check that the cable isn't cut or frayed (especially near the tips), and that the brick warms up a little (a sign it's working). A borrowed charger that works points to the culprit: yours. A non-compliant or underpowered charger can also refuse to charge.
Tip: clean the connector (dust, lint) — a bad contact is enough to block charging.