I was recruited by Corporate and sent to a small Northern Town…

I was recruited by Corporate and sent to a small Northern Town to replace the General Managers friend whom he subsequently had to fire. Needless to say it did not make the transition an easy or welcoming one. Upon arrival I find that the General Manager's son is a salesperson, his wife is the finance manager and his other lifelong best friend is the sales manager below me. My position is directly underneath the General Manager as the General Sales Manager.
After a rocky almost two years, we increased profit by more than was expected. My guaranteed wage went from $150k to over $200. Which proved that I was capable and efficient and was doing my job. The family certainly didn't "like" me and were not friendly on a daily basis but I did my job. Corporate was happy, I was working and bringing home a good paycheck. Corporate had stated in my interview that if I was doing my job in two to three years, I would move up to General Manager when the current man retires. Needless to say he did not want to retire and if he was going to, felt his son and best friend should be moving up. I have a degree, a strong resume and have been a General Manager in the past. My record, my past, my professionalism all speak high. This is the first time in my life I have been treated with such disregard.
My wife and I two weeks ago decided to buy a larger family home. For job security I spoke to Corporate and asked how I was doing, what the plan was for me, should I worry about a transfer. I also let them know that I still wanted to run my own store. Confirming that I was in this for the long haul. I can put up with this family trouble if it means another year but much more than that I wasn't interested in. Corporate agreed to have a conversation and that we would meet after my family holiday in two weeks.
I returned to a text message saying you are no longer employed at this dealership. You spoke to Corporate, you are not to come in here tomorrow as planned. I contacted Corporate who had no idea this was happening. It has been four days of agony with nobody telling me, am I fired? Am I transferred? What is going on?
What are my options? I was told by corporate that while gone, they spoke to the General Manager and he took the credit for increases in profit since I arrived and said I was not doing a great job and not in line for being a General Manager. I have been offered General Manager jobs before based on my reputation. This is devastating to me. Not once was I told I wasn't doing my job or needed to do something else. There was NO communication or guidance.
Please tell me what I could be entitled to or what I should do. I have an employment letter stating two weeks pay for every year and I was three months short of two years but I feel my reputation has been destroyed for corporate with NO REASON WHATSOEVER. What is the minimum I could sue for and what is the maximum?