For Windows 7 or Vista, run Windows Start-Up Repair. Consider a repair installation if you have XP.
If Start-Up Repair was notsuccessful, then consider restoring from your recent full image backup if you have one.
If your computer won’t boot normally, sometimes booting into Windows Safe Mode can open the door to fixing the problem.
Click here for additional troubleshooting suggestions for start-up problems from pcsupport.about.com.
If your hard drive has ‘crashed’ and you are technically inclined, here is a checklist from Intel to help debug the situation and hopefully get your system back online.
Unfortunately, one scenario that can prevent your computer from booting up is that your hard drive has failed due to a mechanical or electrical problem. If that happens, you have to decide if it is worth the money to replace the hard drive. On many laptop computers, the cost to replace the drive represents 25-50% of the cost to purchase a new laptop. If your failed laptop is several years old, you may decide to spend the money on a new laptop instead.