When you think about what an insurance contract is, you need to understand the underlying value. An insurance contract is a financial obligation that both parties must adhere to. Therefore, unless the entire insurance company of Southwestern Life Insurance Company closed in the form of bankruptcy, they still have some obligation to their policyholders moving forward.
The first piece of information you need to know is how they closed. There is some basic information available online stating they merged with Valley Forge Life Insurance Company, however, you should have that contact information available with your original policy documents. If the company did merge, then the annuity contracts will either continue to be paid until that date, or the payment will not start until that date. As long as you have the contact information of the new company, then you can use your current policy information to get the specifics from the new company they merged into.
At the end of the day, the value of the annuity is simple. Assuming there is still a company who has ownership of the contracts (either Southwestern, or Valley Forge, or etc.,) then that company has the obligation to pay. In other words, unless there was a full bankruptcy, someone is on the hook to pay your financial obligations. And, as long as you have some actual financial benefit coming in (now or in the future), then you should be able to sell that obligation on the open market. There should be face value and you should be able to sell your interest in the annuity simply for the fact that any financial instrument or contract can be sold as long as someone is willing to pay you for it. It sounds like you just need to track down who now is responsible for the servicing and payment of the annuity policy, and then you should be set!