Some Golf models typically include an immobilizer activated by a chip integrated into the key. This system is called a transponder, if I remember correctly. Some VW Gols also use this system. Under the steering wheel, on the back of the ignition panel, there's a large black box where you can see two connectors to a module and two thick wires that are part of the immobilizer. Essentially, the immobilizer acts as a switch for this wire, connecting or disconnecting it to start the engine. If your car doesn't have this system, it's very likely that the immobilizer is part of your aftermarket alarm system, and the problem is usually caused by a separate relay. This relay has, or should have, a fuse; perhaps the fuse is blown, or the relay itself is faulty. I hope this information is helpful.