If you can't pull the starting cord, then something in the motor has seized. When the motor is in forward can you turn the propellor and make the motor turn over? There are several things that can cause an engine to seize. If it has ever overheated badly, it may be beyond economic repair. If it's just stuck from sitting, try pouring a few tables spoons of light oil or kerosene, into the spark plug hole, allow it to sit over night and try to turn the motor by hand. You may be able to get a socket on the flywheel nut and rock it loose. PATIENCE HERE.