A LLB occurs when part of the boot process is corrupt -- boot1 is corrupt, or boot2 is corrupt. Since boot2 is required to be working and able load an IOS before the system can run, any errors in this process will render the system inoperable. A LLB cannot be fixed with software and requires hardware modification to be repaired.
Fixing this brick
LLBs cannot be fixed with BootMii, even if it is installed as boot2. Instead, Infectus must be installed into the Wii hardware to install proper boot contents. A NAND backup is not required for this, because boot1 and boot2 are not encrypted with the NAND key.
Cause of this brick
This is one of the most dangerous bricks, because not even BootMii can fix it. While boot0 is in ROM, boot1 is on the NAND and is SHA1 hashed in OTP. This means if boot1 is modified at all, the signature check will fail, and boot0 will refuse to boot. boot2 suffers in the same way, although its signature is checked via RSA signature verification instead of ROM, so this brick could also be caused by a corrupt boot2 signature, resulting in boot1 refusing to boot the system. No part of the official boot process allows recovery software to be booted, leaving the Wii in a bad state.
Many low level bricks were caused by the 4.2 update incorrectly overriding boot2.