Small Engine Troubleshooting Problems? Ask a Mechanic Now.
the flywheel on the mower is indexed with a soft "woodruff" key to try and prevent engine damage.
When you hit an object like the tree root this would cause the key to shear. You will need to remove the engine flywheel and replace the key. You will need a special flywheel puller that you can buy from a lawnmower repair shop or you can just have them fix it.