Physical failures

• When computers start, they go through a power-on self-test, or POST. The manufacturer of the system builds in a series of hardware checks that must pass before the computer can reliably load an operating system. If the boot volume, or the hard drive itself, fails, it manifests as a hardware failure during the POST. Misconfigured BIOS settings can prevent the system from starting.
  
  

Software Errors

• Assuming that the hardware passes POST, the next thing to look at is potential software corruption. The operating system loads after the POST. A wide variety of issues cause loading problems. The critical start-up files needed to load the operating system itself may be missing, or drivers may conflict with each other.
  
  

Viruses or Malware

• Viruses or malware also cause boot failures. The master boot record, or MBR, is often a target of viruses and malware. The MBR tells the computer where on the hard drive the operating system files are located. Viruses attacking the MBR corrupt this data, preventing the computer from starting up. Boot/Stoned.Monkey was an early virus that attacked the MBR.
  
  

Hard Drive Recovery

• Bootable CDs, USB drives and floppies allow users to regain access to computers. If the hard drive itself has not failed, then the data or even the operating system itself can be recovered. Hard drive manufacturers, operating system vendors or other software companies often provide utilities to assist with recovery efforts.