There is a small metal piece called the flywheel key that is cut into the flywheel and crankshaft. This key is a safety feature so that if your MasterCut push lawn mower hits something hard like a branch, root, or rock the flywheel key will shear in half. When it breaks, the flywheel will not rotate in synch with the crankshaft, preventing the engine to run or start. It was designed this way to prevent major damage to the engine.
If the flywheel key is broken or sheared it will need to be replaced. It may be a good idea to have a couple of them on hand just in case it happens again, they are small and inexpensive.
You can find the model and serial numbers located on your MasterCut push or self-propelled lawnmower somewhere on the deck of the mower.
NOTE: if you need a part for the small engine in your MasterCut push or self-propelled lawnmower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your MasterCut push or self-propelled mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: