The first step is to call your own insurance agent and report the accident. They may be able to recognize the policy number from other customers they’ve worked with, or they can do some digging to help you find out.
Another option is to check with the DMV. You might be able to visit or call your local branch and have them look up the company with the policy number. Make sure you have some identifying information with you, as well as a copy of the police report, so that you can more easily explain the situation you’re in. Once you’ve explained and shown them the necessary information, they should be more than willing to help you out.
Good luck tracking down that company!