Why is the Mini Display port on my macbook air 2014 not working?

I have a 2014 macbook air. I have been using it frequently in clamshell mode (computer closed with external monitor, keyboard, mouse, and display) and it has been working great. Until, the mini display port just stopped working. I purchased a new HDMI cable and a new adapter just to rule those out and it is not the problem. I also check the monitor and cables with a windows computer and they seem to be working fine. Does anyone know why this port might have stopped working and how I could fix it? Apple Tech support was not much help...