Startup-related problems can point your computer troubleshooting efforts in multiple directions that lead to hardware- or software-related failures. Assuming you didn't ignore the low battery warning and deplete the battery, a portable Mac that shuts down spontaneously and won't restart may need anything from a new OS installation to hardware repairs. Some of these tasks fall outside the capabilities of users who lack access to a full set of tech tools and the expertise to use them. To pinpoint the source of your Mac's malfunction, take advantage of Apple's built-in diagnostic options and reset procedures.

Corrupted Operating System

If the files that make up your operating system become damaged, they can prevent your Mac from starting. In some cases, you press the power button and see a flashing question mark on your screen, When the startup sequence won't proceed past that onscreen signal, your Mac laptop can't find a viable installation of OS X, either because of file corruption or hard drive damage. If the startup process moves to the next step, you see the Apple logo and a spinning progress indicator. A corrupt OS can halt the sequence at this stage, too. To troubleshoot your system, start up from your Mac's hidden Recovery System partition or -- with an Internet connection -- the online equivalent. Press "Cmd-R" and hold down the keys as you start up the computer to access either recovery mode.

Battery Problems

Without a reliable power source, your Apple portable can't boot or function correctly. Unlike older portable Macs that used removable batteries, current Mac notebooks incorporate built-in power cells that you can't replace yourself. If you have access to a spare MagSafe power adapter that matches your computer model, see if your MacBook or MacBook Air runs and your battery charges properly with an alternate power source. To replace the battery, contact an Apple retail store to make an appointment at the Genius Bar.

Hardware Defects

Hardware-related problems can be both disruptive and difficult to diagnose, especially without specialized service equipment. If your system uses a traditional platter-based hard drive that ticks, howls or whines, these unusual noises signal impending mechanical failure. SSDs, or solid state drives, feature no moving parts and can fail without making noise or giving any warning. Startup failures also can point to your system's logic board, power supply or RAM. Memory failures may cause your Mac to beep when you try to start it up, signaling its problems in a diagnostic code. To test your computer, disconnect all external peripherals and start the system while you hold down the "D" key. This startup command forces your system to boot from built-in diagnostics or to use your Internet connection to load the online version of these test routines directly from Apple.

NVRAM Reset

Macs contain several components you can reset through key sequences that you press and hold as you start your computer. An NVRAM reset clears the values stored in a small battery installed on the system's logic board to maintain some aspects of startup information, including audio volume, startup disk and other parameters. To clear NVRAM, press and hold "Cmd-Option-P-R" before you hear the system's startup chime. Hold down the keys until your Mac chimes three additional times. If your system boots successfully after an NVRAM reset, open System Preferences from the Apple menu and check the startup disk and audio settings.

SMC Reset

Problems with the System Management Controller can interfere with indicator lights, startup sequences, video and application performance, and cooling-fan speeds. SMC reset procedures changed when Apple stopped building portable Macs with removable batteries. With your system shut down, connect its power adapter to an electrical outlet and the Mac. Simultaneously press the power button and the "Shift-Ctrl-Option" keys on the left side of the built-in keyboard. Release the keys and turn on the computer. On an older system with a removable battery, turn off the computer, unplug its power adapter, and unlatch and remove the battery pack from the bottom of the case. Press the power button and hold it for five seconds. After you release the button, reinsert the battery and reconnect its adapter.