cant boot os x after installing windows

I have a 2011-2012 macbook pro 15. yesterday I installed windows 7 using bootcamp and installed all drivers. everything seemed fine and I even switched to OS X Yosemite once and went back to windows again. and then I tried to restart and boot back to OS X and it did not let me. so basically once I restart, I hold the option key and when I do that nothing happends. all I see is a black screen. it does not show me the booting options. If I restart the computer and touch nothing windows automatically boots up. so windows is fine. so what I tried to do is go into bootcamps control panel and change the start up boot to OS X and restarted thought this might automatically boot osx but it didnt. so macbook got restarted and it stayed on black screen. option key still doesnt work. I also tried the command + R and i get the utility screen but I still cant boot OsX eventhough i can load windows again from start up on my utility screen. so I dont know if i need to re install OSx, or if its the drivers or a hardware issue or what. please help me out, I need my OS X and files that i have on it