I've been working in the restaurant field for over 20 years, so I seen a lot of restaurants come and go.
If they went out of business because of financial troubles, I think it might be hard to get any money out of them.
Were they part of a conglomerate of restaurants? If so, maybe the sister restaurant would honor the gift certificate.
You can always try to contact the owners, and see what they say. It doesn't hurt to ask.