MJ7 is the bootloader that powers CM currently. You need a bootloader to be able to boot into the operating system. Like a key into the phone.
If you upgrade your bootloader to a newer version aka NA6 it locks the phone. Therefore anything that runs lower than NA6 aka MJ7 won't work.
Think of it like, you broke your door, you got a new door and tried to use the old key. It no long works.
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