Radio controlled cars are a popular hobby for car enthusiasts. They do have many moving parts and like their real life counterparts, can develop faults. Cars that won't move forward may be suffering from a number of issues ranging from a broken engine, a drained battery or faulty gear casing.
Check the engine by removing the two screws that attach it to the gear case.
Gently lift the engine out of its mounting. Try to turn the motor using your fingers. If it doesn't move, you may need to replace the engine.
Check that the motor-driven tires turn correctly by holding them and turning them manually with your hand at the same time. If they don't turn fluidly the gear case may be at fault. Have the case inspected by a specialist.
Check that the batteries are fully charged by leaving them in the charger unit until it signals that they are fully charged.
Check that the fuse in the electronic speed control has not blown by using a voltage meter.