Try standing vacuum on end, then gently tugging and releasing cord. This generally will retract it.
There are many problems that can result in disassembly and reassembly by an untrained person, including shock and fire. For those and other reasons, I don't recommend trying this yourself. Take it to the nearest Aerus Electrolux store for repair.
The back of the vacuum can only be removed by separating the top and bottom portions of the canister. The cord winder will need to be removed and the wires all disconnected. The cordwinder is then taken apart and the cord rewound and set for proper tension for rewinding. If the cord is simply stuck due to having been pulled out too far, this is generally not necessary.

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