If you are running v10.7.x Lion use Lion Recovery to repair the startup disk if necessary, or reinstall the Mac OS X.
For Mac OS X prior to V10.7 try here > Resolve startup issues and perform disk maintenance with Disk Utility and fsck
When you post for help, please tell us which Mac OS X you have installed
When it boots, does the hard drive make a clicking sound? If it does, try taking the battery out and putting it back in. It could just be that the hard drive doesn't have enough power to work properly.
If that doesn't work, restart. When the white screen appears, either press and hold Command+R or the Option button. When the boot screen comes up, select Recovery HD.
If none of the above work, it is most likely an OS error, and reinstallation may be necesary.