Little Snitch consists of multiple parts, some of them operating at a low level of the operating system, called the kernel. Little Snitch needs to trigger an update of kernel caches and requires a reboot during installation. Under rare circumstance this kernel cache update might fail, which may prevent your computer from starting.
If this happens during the installation of Little Snitch on your computer, please start it in safe mode by holding the Shift key (⇧) (you can find more information about safe mode here). If you are using OS X Yosemite or earlier, please repair your disk permissions using Disk Utilities.app (for detailed instructions see How & Why to repair disk permissions in OS X). As second step, start the Little Snitch Installer once again and perform a restart as instructed.
Usually this solves the issue. If you are still unable to start your computer, please get in touch with our tech support team.