While there are many types of failures that can prevent a system from booting, the same basic procedure can be taken to bring the system up. When a system fails to boot, you should first try to identify the failure by the evidence left behind on the screen and then attempt to repair the problem (for example, by turning on a drive that was accidentally powered off). If the problem is one that cannot be repaired (such as data errors on the boot disk), boot the system from an alternate boot disk that contains a mirror of the
root volume, so that the damage can be repaired or the failing disk can be replaced.
The following are some possible failure modes: