Mac will not restart properly with windows in bootcamp

I have a 15in Mac book pro Retina running yosemite. I have partitioned and installed windows 7. I use it regularly and it has been working fine for the past two years. It runs updates and successfully starts and restarts when switching between OS and Windows. However, recently, when I restart my Mac (switching to Windows bootcamp), my computer will restart, show the windows login screen for a split second, and then appear to shut down. (The screen will go black, but the keyboard stays lit.) I have tried force shutting down and restarting to no avail. I have also tried holding down option during restart and troubleshooting, but it also does not work. I have tried waiting and not pressing anything to see if it will fix itself, and it does not. Has anyone had a similar problem or know what is going on here? Please help

-KHB