Macbook doesn't find boot drive with OSX anymore..

Hello,

I have a macbook where I replaced the optical drive with an SSD (crucial M4/64Gb). OSX is installed on that drive.

The original drive is formatted and windows 7 was installed on it with bootcamp.

This worked fine for over a year. I installed simcity 5 on windows 7. No problems. But yesterday after playing the game for over 6 hours, I couldn't boot my macbook anymore. Blank screen, then a folder with a question mark commes on the screen (the no bootdevices found logo).

When I hit 'alt' when booting, only the windows hard drive appears, and I can boot into windows fine.

In windows itself, my 'macintosh HD' drive is also gone.

What's wrong??