Mac OS is typically a stable operating system, but it can experience certain errors that make it unusable. If your MacBook is stuck on the loading screen and will not continue no matter how long you wait, it may not be apparent what is causing it. If the problem is caused by software errors, you can troubleshoot and correct the issue by resetting certain parameters and attempting to repair your disk and permissions.
Your MacBook's PRAM (parameter random access memory) contains sets of data that are retained after your computer is shut off, such as your autokey rate, alarm clock settings and speaker volume. Turn off your computer, then turn it on again -- immediately after pressing the power button, press and hold the "Command," "Option," "P" and "R" keys simultaneously until you hear the start-up chime go off a second time.
Reset System Management Controller
The SMC is also responsible for certain low-level settings retained while your computer isn't in use. The data it retains is different, and the process for resetting it quite different. Shut down your computer, unplug the power adapter and remove the battery. Press and hold the power button for five seconds, then release it. Replace the battery, plug in the power adapter and try starting up again.
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Repair Permissions & Disk Errors
If the problem is being caused by a software-related issue, repairing your disk permissions and fixing any errors can help correct it. These actions can be performed through Apple's Disk Utility, which can be loaded via your Mac installation disc. Insert the disc into your computer, then hold the "C" key as the computer starts to boot from the CD instead of your operating system. Choose your main language, then press the right arrow icon. Click "Utilities" on the top bar, then click "Disk Utility." Select "Macintosh HD" from the left side bar, then click "Repair Disk Permissions" to scan your computer for any faulty permissions; they will be automatically repaired. When this is finished, click "Repair Disk"; this process looks for any actual errors in the system, and also repairs them automatically if possible.
If none of the above processes work, the error could be caused by damaged or faulty hardware, such as the logic board. If this is the case, your MacBook will need to be taken in for servicing at an Apple store (or another computer repair shop if your warranty has expired).