Are you 100% sure the client is completely gone - is there not some bankruptcy process in place that tries to divide the assets fairly among all creditors? If you sell the solution to someone else, you may be forfeiting any chance of getting anything out of the bankruptcy, which might still be the easiest way to get paid.
If there is absolutely no way for you to get your money, I guess everything you do then depends on the nature of the solution.
If it can be re-purposed and sold to someone else, obviously that would be the best way to go because you can actually earn something out of it after all.
Maybe it's also a good candidate for publication as Open Source - if it's a high-quality, well-documented product, and if you are ready to put some serious time into documenting it and "getting it started". Publishing something properly as Open Source is a lot of work, and nobody needs another dead rotting project on Sourceforge. But it could benefit you in many ways in the long term, even though you wouldn't be making any money out of it initially.
Maybe if you add some more detail about the solution and its target market, somebody can give you some more specific advice.