A past due invoice of $18,377

Thank you for your assistance. I have a small turfgrass consulting firm in California. We evaluate new products (fertilizers, organic products) to be used in the home lawn, golf course and sports field market in California. In 2014 a firm from Minnesota retained our services to evaluate their new product on creeping bentgrass putting greens in Seaside, CA. The cost to complete the work was approximately $33,000. To date, two years later they still have an outstanding balance of $18,377. They have received 19 past due notices and numerous phone calls. They are no longer responding to my requests for payment. Where can I go from here in order to try to recoup this past due total? Where do I start? What are my legal options? What are the next 3 things I should do immediately to address this issue? Please help me! Thanks so much.