Depending upon the type of lawnmower you use, many factors contribute to your lawnmower not starting. A gas power mower might have run out of gas, have a weak battery, or maybe short on oil. A corded electric mower might have a bad connection to the socket, or there will be any wire breakage in the extension cord. A self-propelled lawn mower may have a nonfunctional recoil cord; a cordless electric lawn mower may have run out of battery or a poor connection between the motor and lawnmower. If the issues are such that you are unable to fix yourselves, seek professional help.