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symbioidljYour first step should be to check the hardware, which can fail or simply get corrupted. For example, a "wild write" by software can cause invalid data to be stored in the Mac's hardware Parameter RAM. The PRAM includes the data that determines which device the OS uses to start. Simply resetting the PRAM to default values can sometimes correct a boot problem. Resetting the PRAM is easy: hold down the Option, Command, P, and R keys at the same time while restarting the machine until the startup chimes sound. If the PRAM was the problem, the Mac will now boot normally and load the OS. You can then personalize the default PRAM values.
