I put 2-stroke fuel in my 4-stroke mower and now it won't start
Remove the air filter and spray a small blast of starting fluid into the carburetor. It should run for a second or two. If it does then the problem is the gas is most likely old/bad. If it doesn't run for even a split second then you're not getting any spark which means you have an ignition problem which is most likely a fouled spark plug.