When you start up your Mac, the system will generally show a gray Apple logo for a brief period followed by the log-in screen or other user interface elements as the rest of the operating system loads. In some instances, after crashes, power outages, or changes are made to the system, instead of booting properly the system might display the Apple logo and then reboot, repeating over and over.
Once the OS X kernel loads properly, even if there is a problem with background services, the computer will still boot but will just display unwanted behavior such as location services or Spotlight not functioning. Therefore, if your Mac is constantly rebooting after displaying the Apple logo, then there might be a problem with the kernel extensions that is preventing the system from booting properly.
In the OS X boot process, when the Apple logo is showing, the system has found a valid boot device and will then display a spinning wheel below the Apple logo when the system loads the kernel and its extensions. At this point, the system can load kernel extensions from several sources including boot caches and from the extensions' locations on disk. In doing so, odd problems with the files from any of these sources could result in problems that force the system to restart.
To tackle a repeated reboot, the first step is to see if you can get the system to load properly in any Mac operating system environment. First try booting to Safe Mode by holding down a Shift key when the system restarts. If the computer is able to boot properly, then you know the issue is likely with some third-party extension or add-on you have installed. This is especially true if the system only boots to Safe Mode and continues its reboot loop when you restart without the Shift key held down.
If you cannot boot to Safe Mode, then try restarting to another OS X installation such as that on an external hard drive or the OS X installation disc (Lion users can boot to the Recovery HD partition by holding Command-R at startup).
If the system shows the same behavior when booting to a different boot drive (especially to the OS X installation drive), then it suggests a hardware fault is to blame for the problem, and not something to do with a configuration of the operating system. In this case, you can try resetting the PRAM and SMC for your system, but beyond that you will likely need to have the system serviced.
If the system does boot properly in Safe mode, and also is able to load when using a secondary OS X installation or boot disc, then the problem you are experiencing is likely with a third-party kernel extension or other system add-on that you might have recently installed or configured. One easy way to figure out if this is your issue is to use the utility Pacifist. In its latest version it includes a kernel extension scanning utility that will display information on the kernel extensions installed on your system.
Download Pacifist from the Charlessoft Web site, and open it (you will have to wait 15 seconds for unregistered versions), and then choose Display Kernel Extension Report from the Pacifist menu. The program will scan through the Apple-related installer receipts and match whether an installed kernel extension was included with OS X or from a third party.
Using this report, you can easily identify the non-Apple kernel extensions and be able to remove them to see if the system will load correctly with them uninstalled. Pacifist's report offers a Reveal in Finder button that allows you to quickly locate the extension, or you can try identifying the program associated with the extension and run an uninstaller for it, if provided.
If you are unable to load the system in Safe Mode but are able to boot to an external drive, then you will have to take another route to address the issue.
As a separate step in this instance, you can try identifying possible causes to the problem by booting to Verbose mode, which will show which drivers and system components are loading in the background, that is, until the system loads or attempts to load the user interface. To use Verbose mode, hold the Command-V keys together when the system reboots, and instead of the grey Apple logo during boot you will see scrolling text. Watch the text and if it pauses at a certain point right before the system restarts, then try to write down any information about the last few lines you saw before the error occurred, so you can convey them to a technician or further investigate yourself to see if the problem can be fixed. In some cases the output might reference a clear error about a specific OS or hardware component, though in other cases the output might be a bit more cryptic.
If you are able to identify a specific kernel extension that is causing the problem, then you can boot the Mac into Target Disk mode and access its drive from another Mac via a Firewire connection, and then browse the drive for the extension and remove it from the Extensions folder.
Unfortunately this type of approach does require you to have two Macs, with both having FireWire connections, but is one way to remove a faulty extension from the system so you can at least be able to boot it again.
If you cannot address the cause of the problem by uninstalling applications or managing third-party kernel extensions and add-ons, then a final approach would be to reinstall OS X over your current installation, which can be done by simply booting to your OS X installation volume (be it a Lion recovery partition or an installation DVD). Follow the on-screen instructions to reinstall the OS. If you have a Time Machine backup of your system, then you can optionally restore to a backup made before the problem started happening (a preferred route). If not, then a full OS reinstallation is the next best approach. Doing this should keep all of your user data and installed applications intact, and only replace the operating system files underneath everything else on the drive.