It can means a lot of stuff.
1- You are not using Version 3.1.2
2- You exit / turn iPod off / Disconnected cable while jailbraking.
3- You are using Mac Version
4- You mess-up some other way
For some reason I had the same exacly problem with my laptop; so I borrow my dads and it worked just fine, in fact I am writing while on my phone...
Pleas, give me more info about what is happening and showing... I might be able to help you further.
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