In our experience, most problems with Mac’s not booting at start up are associated with hardware. The majority of these are related to a failure of the hard drive, but there are also occasions when the failure involves the logicboard (motherboard), RAM, power supply, or graphics controller.

Replacing a hard drive is relatively straightforward exercise, though not necessarily a cheap one.

The Mac OS X is a relatively stable operating system and in our experience, less problematic than Windows and thus less likely to result in failure to start up. Nevertheless a poorly maintained, out-of-date system, or even one repeatedly “updated” from one version to the next over a period of years, can eventually lead to sudden problems one day with the result being that your Mac won’t boot.

In this instance, you’ll be relying on your Time Machine backup to get you up and running again.