This could be if there was a power outage/fluctuation, an app crashed, you ran out of HD space (yes, that’s happened to me, I download… a lot) or if your system libraries are corrupt.
Once the Kernel loads properly, even if there’s a problem with background services like location, the computer will still boot, only in this case those particular background services may not work. This can be reset with a simple reboot. However, if your computer is not booting, but is stuck in a boot loop, then there could be two problems : it could be a kernel issue, or a hardware issue.
To narrow it down, lets take it one step further. During the boot, when the Apple Logo shows up, that means the System has a valid boot device (ie could be your bootcamp, or your HD, or your bootable HD). Then a spinning wheel shows up underneath. At this point, the system loads kernel extensions, boot caches and more. If one of these are causing a problem, your reboot should start immediately after the spinning wheel. If not, then it could be a add-on causing the problem.
To perform a PRAM Reset:
Ensure the machine is powered down
Locate the following 4 keys on the keyboard: Command, Option(Alt), P & R. You will need to hold these keys down simultaneously
Press the power button
Press and hold the Command-Option-P-R keys. You must press this key combination before the grey screen appears.
The machine will present the Apple logo and reboot – You want the machine to reboot 3 times(Apple chime 3 times) so keep holding the keys!
Has this worked?
If not, to perform an SMC Reset:
Ensure the machine is powered down
Unplug the computers power cord from the machine
Wait 15 seconds
Attach the computers power cord
Wait another 5 seconds and press the power button to turn on the computer.
Also, try this:
Ensure the machine is powered down
Press the power button
Hold the SHIFT key down – this must be pressed before the grey screen appears.