Depending on what notice was provided to you, you may have a cause of action against the lender for violating the state's laws for repossession.
(See this publication for a helpful guide on what notice is required, and what your rights are: http://greatexpectations.vccs.edu/wp-content/uploads/Repossession-Deficiency-Judgments-2-6-2.pdf).
If you were behind on your payments (even just one payment) they were entitled to repossess the vehicle, but they should have given you the opportunity to reclaim the auto by making full payment prior to sale.
If they did give you proper notice (again see the link above for details), and you did not make payments, your next step is to try to negotiate the deficiency.
When trying to settle a debt, creditors generally prefer lump sums over payment plans. They are often willing to accept an amount less than the full debt (the trade off is that they get a quick payment and don't have to worry about ongoing collection costs or administration). If you do not have the ability to offer a lump sum for something the creditor will accept (some will accept a small portion, while others want close to the full amount), you can try a payment plan, these are less satisfactory to the creditor (especially if they have a lien on your property already), but if you are willing to offer something with a reasonable chance to get the creditor a large amount of their debt back, you are likely to get them to accept it.
Whenever working with a creditor, make sure that you keep your communications in writing (if you speak to someone by phone, promptly send a confirmation letter to summarize your conversation), as this will help to ensure that there is no confusion later on, and you will be able to enforce your settlement against any future collection efforts.