Troubleshooting power-up issues for Mac systems
If your Mac does not sound the boot chimes and load OS X when powered on, then there is likely a hardware fault at play.
Most troubleshooting for Mac systems revolves around software-based problems, be it bugs in the system software or a faulty OS configuration or installation; however, sometimes errors can extend beyond the operating system to the hardware itself, and when this happens, you may see no activity when starting up your system. However, even though the system may appear dead, it might have some indications of the problem at hand.
In thethe first step that occurs when you press the power button is all components such as hard drives, video cards, I/O controllers, and add-in cards power on and begin initializing their firmware, followed by assuming a ready state to wait for instructions from the system. The motherboard's firmware executes and loads from the system's read-only memory (ROM) where it runs a basic power-on self-test (POST), which is a quick assessment of the system's capabilities before sounding the boot chimes, loading the EFI firmware, and continuing the boot process to pass control to the operating system software.
In the process from power-up to POST, the system will not display anything on the screen, but may give an indication if there is an error.
The very first indicator that your system is functioning properly is if you see or hear any activity with LEDs and fans in the system. When you press the power button, you may hear fans whir and see LEDs light up. If this does not happen and you see no activity within a few seconds after having pressed the power button, then it suggests either the power button is broken, the power supply is broken, or the system management controller (SMC) is corrupted and not working correctly.
While broken hardware will need to be fixed by a technician, if the SMC is corrupted then you can perform an SMC reset on your system to clear the controller's firmware and have the it properly manage power distribution to system devices. The procedure for resetting the SMC depends on the specific system you have, but Apple has a knowledgebase document that covers how to do this on various systems.
Beeps and flashing LEDs
If you start up your Mac and instead of hearing the boot chimes you get a single loud beep or series of beeps, then this indicates a hardware failure either in the processor or in the RAM. A single beep indicates no RAM is installed, which indicates the RAM modules are not getting enough power or have become corrupted to the extent that the system does not register them as being present.
If you hear two beeps, then this indicates you have RAM installed but it is determined to be incompatible with your system. This issue is likely the case after you have upgraded your RAM with modules that are too slow for your system's RAM bus. If you hear three beeps, then the system is telling you the RAM is recognized but for some reason it cannot access the memory banks, which is likely the result of corruption from static discharges or factory faults in the memory chips. In these situations, your best bet is to replace the RAM.
If you experience more beeps than a series of three when powering on your system, then this indicates a problem with your computer's ROM or processor, and it will need to be taken in for servicing.
In addition to beeps, the system may flash its power LED in various patterns to indicate similar issues. If you see groups of pulsing LED flashes, then this indicates your system cannot recognize a proper amount of installed RAM, and if the LED is pulsing slowly then the RAM is either bad or missing. Additionally, the LED can indicate a boot ROM failure if it is on and there is no other activity on the screen.
No activity after boot chimes
A final issue if you hear nothing after the power is turned on is faults in the system's ability to load parameter RAM variables, which are hardware settings the system uses before it loads the system software. While PRAM errors usually result in odd setting being persistent after bootup, on rare occasion a fault in the PRAM may result in a hang when the system tries to access it, and clearing the PRAM is the only way to get around this issue. To do this, restart your system with the Option-Command-P-R keys all held down at once, and release them after you hear the system reset and sound the boot chimes again.