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.

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. 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. 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. When you press the power button, you may hear fans whir and see LEDs light up. 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. 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 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 Mac's startup sequence the 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. However, even though the system may appear dead, it might have some indications of the problem at hand. 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.

Macbook pro 3 beeps without opening up