If your lawn mower will not start how you repair this depends on the type of engine you have. For example, there are two main types of engines: fuel and electric. Below are problems you can have with each of these, so you can get your lawn mower working again.
If your lawn mower's engine runs by fuel and it will not start the first thing to check is the gas. For example, if your lawn mower has set for a long period of time the gas may need to be replaced with new gas. Empty the tank and put high-quality gas in it.
It could be due to a dirty carburetor. You can purchase carburetor cleaner at a home improvement store or online. This cleaner is generally in a spray can. Follow the directions on the cleaner on how to use it. If you still see built up dirt on the carburetor use a damp cloth to clean it. If you are not sure where the carburetor is located, refer to the owner's manual for your lawnmower.
If your fuel lawn mower still will not start, you likely have a bad engine. In a case like this, your engine may be able to be repaired, or it will need to be replaced.
With an electric engine, the problem could be with blown fuses. Check these and replace fuses if you find any blown. Your lawn mower should now start.
If you do not find a problem with the fuses, it could be the battery. Before you do anything, disconnect the battery from power to protect yourself from electrical shock. Check the clamps to see if they need to be tightened. The battery terminals may be dirty. If so, the battery cannot make a good connection. Wipe the terminals off with a damp cloth. Reconnect the battery and start the mower. If it will still not start the battery could be dead.
If you are still having problems check the electrical connections, such as for broken or frayed wires. Be careful if you see a frayed wire as this can be dangerous to work with
If you continue to have, problems take your lawn mower to a lawnmower repair shop. They can determine what the problem is and decide if it will be worth it to you to repair the mower. If not, the repair shop can give you tips on purchasing a new lawnmower.