I was hired away from my large, Fortune 500 company to work for a small, local company because of my business experience. I was promised salary, commissions, benefits and, ultimately, a percentage of the company. I received everything but the piece of the business and every time I broached the subject and tried to get it in writing, my employer hedged, postponed, and avoided the subject. After nearly 5 years, we had a "come to Jesus" meeting where my employer completely reneged on the promise. Our relationship deteriorated and I was ultimately terminated. I feel I performed my part of the bargain. I worked long hours and nearly doubled the size of the business because I thought part of it would one day be mine. Nothing was ever put in writing but people know of the arrangement.
This is the type of dispute that you should seek the guideance of a licensed attorney in your state. Depending on state law, you may be able to recover from the prior employer for breach of contract. Undoubtedly you will need to provide every supporting bit of evidence that the partnership interest was offered as an inducement to the employment compensation.