Macintosh suddenly stuck at loading screen?

Hey there,

So I came back from school and wanted to continue with my work.

I start up my Macbook and all of a sudden it stuck at the loading screen.

The process:

1. I pres the power button

2. I hear the sounds and the apple appears.

3. Then there's a loading bar which is only seen for 2-5% of the loading, and which then disappears.

4. After that the spinning wheel pops up and it remains like that.

I shut the laptop down in the normal way.

Also, there's nothing wrong with the hardware since it boots up into Windows 7 via bootcamp without any problems.

I allready resetted the VROM or whatever it's called.

Thank you for your time and help!

Greetz,

Finn.