You can try starting in safe mode, power up the machine and press Shift key until the spinning gear or whatever it is appears.
Another thing you could try is to boot by pressing Apple -key and S at the same time, again keep pressed long enough. Then you see black screen with text, write
/sbin/fsck -fy
And press enter, it will check and try to fix your hard disk (not physically but for file system problems etc... heh)