If it is constructed anything like a Jack Bass (headless) it is a steel rod in an aluminium u-channel.
The usual answer would be to take the fretboard off and replace it.
I bored out the hole to the adjuster nut so that the rod assembly could be extracted, and they come out quite easily if you can grab it with something. This might not be as easy with a headed bass, as the, er, head might get in the way.
Cheap replacement neck might be the way to go...