Upvote:3
Contact your country's embassy or consulate to report the issue, as officially the passport belongs to the country, not to you. Most embassies will not do much directly on your behalf, but most do have lists of suggested lawyers they can give you, through which you can pursue legal action.
You can contact the Royal Thai Police and file a complaint. But if it is a he said you said issue, they tend to side with the Thai side of the story.
Ultimately it will come down to what agreements were made when you were hired. And then weighing those agreements against Thai laws.