Ask HN: Help with Stock Options of Acquired Company

I need some help regarding my options (this was the first time I held startup options). The company I was working for gave me (ISO) employee stock options. They are all vested already. About 4 months ago, this company got acquired, in what I suspect was a down-round. However, I got absolutely no information about this, and don't know the details beyond what was in the press. I tried to contact the new CEO several times, but he hasn't replied to any of my emails. The company I worked for was based in New York and the acquiring company in San Francisco.

It is possible that my options are worthless. However, what is the official/legal procedure here to determine what they are worth? They haven't tried to get in touch with me, and haven't been responding to my emails either. I have all the signed PDFs from the former company/CEO outlining the option details.

Would greatly appreciate any help.