Likely cause is voltage drop from an undersized extension cord. Using a cord with too small a wire gauge (too high a number) causes power loss and overheating.
Fix: Check your extension cord's length and amperage rating. Match it to the gauge table. For a standard 120V tool, if your cord is 50 feet long, you need at least 16 AWG wire. If it's 100 feet, you need 14 AWG. Use the next heavier gauge if in doubt. Plug the tool directly into a wall outlet to test. If power returns, your extension cord was the problem.
If that didn't work, the battery pack may be depleted or faulty. Try a second, known-good battery pack from your kit. If the tool works with the second pack, charge the first one. If the charger indicates a problem with that pack, it may need service.
Heads up: Do not force the tool. Using the wrong power source damages performance and can overheat the charger cord.