Finding yourself behind on your car loan does not mean necessarily that you will lose your car to repossession.  As long as you are not severely behind (like months) you still have some options that can help you out in a jam.

Evaluate the car. If you can get by without the vehicle, then attempt to sell it.  If you can get out of it what you owe, then you are free and clear.  Be sure to communicate with your financing company what you are attempting to do with the vehicle so that they will be aware that you are trying to bring the issue to a resolution.

Lease options. If you are in a lease, then you will need to add up all of the costs that are associated with breaking the lease agreement.  Call the leasing agent and find out if they are willing to re-negotiate with you.  The value of the vehicle and the attitude towards keeping you in the lease will play a part in whether you are successful or not.  Part of that will be determined on what kind of customer you have been in making your lease payments.

Loan options. The good news with a loan is that the financing company stands to lose money if they have to repo the vehicle.  They would rather negotiate and get something rather than take the loss.  Even if you owe more than it is worth, you should still find a willing lender.  You might be able to re-finance the car at a better rate with a different lender.  The caution here is to try to avoid a long term – make it as short a term as possible.  Be sure to check out the local credit unions in your area.  They often have the best rates and are more willing to work with customers because of their focus as a lending institution.

Extensions.  If you are just a month or so behind, you might find the lender willing to grant you an extension on your loan.  What this will do is to extend the terms of your loan by a month or two and get you current.  Keep in mind, however, that these programs are limited and you can only apply for them only about once per year.
Hopefully you can find a way to keep your vehicle and maintain your credit at the same time.  This is a win-win for all concerned and will help you get through a tough time.