Not sure if this helps, but I read this in the controller release notes:
CSCsd52483?When you make changes in the bootloader of a 2006 controller or a Controller Network Module, the bootup process may halt, and the controller may stop responding. The controller also displays the "grub>" prompt on the console port.
I had the same issue, but found different solution. In my case, CF card inside 2106 came loose. In order to re-insert it, top cover has to be removed. It is fairly easy with 3 screws on the bottom of the unit.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
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