To start off I'm not great with technology but I try to learn. Okay so I installed Microsoft vista to my mac air using bootcamp I have no idea how I did it but anyway I ended up not using it and just left it for a few months, all of a sudden only windows starts, mac won't. every time I hold down options trying to switch back to mac the screen just stays black but when I just turn it on it goes straight to Microsoft again. I have also tried holding c when starting up and tried holing control option command shift. All have the same result of the screen just staying black. Please help me I have no idea what to do?