First you will need to perform a disk repair. To do this, reboot the computer and continuously press F8. Select Advanced boot menu then choose Safe Mode with Command Prompt. Once the Command prompt appears, write the following command:
chkdsk /r. Wait for process to finish then restart the computer. If it still doesn't want to boot, press F8 again and go to Recovery. Select Startup recovery and let it scan. If everything works good, Windows should boot. Additionally, you might need to reinstall Windows again by performing an overwrite using the install disc/USB device.