PRAM contains Startup Disk settings that may need a zap to fix some booting issues not related to hardware. Start up your computer holding the Command-Option-P-R keys, then keep them held until your computer restarts and you hear two startup chimes (release them when finished). If your computer boots normally, check your settings in System Preferences—many were probably reset.

Safe Mode doesn’t look different from Regular Mode, but it may get your Mac booting again.

Safe Mode makes a great diagnostic tool, and sometimes it gets sick Macs booting by itself. Start up your Mac holding the Shift key and let go when startup kicks in. OS X will run with a minimum of features and extensions to make troubleshooting easier. Safe Mode also performs a directory check of your startup disk, among other diagnostics, which may be enough to put your patient back on its aluminum feet.