Join Date: May 2009
Location: South Carolina, USA
The Night Lord in question in Void Stalker is named Mercutian. He is hit directly in his powerpack by a thunderhammer (causing it to explode and sending him to the decking for a time), but is able to stand and move after the fact with the note-
'... Without power, his armour was almost useless, adding nothing to his strength or reflexes. The joints didn't whirr, and the backpack didn't hum.'
(I'd give a page number but apparently my ebook copy is disinclined to provide me with one at this time).
It is also pointed out shortly after the above quote that Mercutian also is no longer tied to any vox and/or squad uplinks so he removes his helm to communicate with his brothers as the slowly make their way through their ship, indicating that he is able to move in armour that is totally devoid of power as well as heavily damaged.
Movement in damaged/deactivated standard power armour is possible though not the most elegant of things, however, I recall reading at some point that it is all but impossible to move in unpowered terminator armour due to the sheer weight and size of the plating. Note that when I say 'all but impossible' that does not mean that you won't find some mention of a champion astartes being able to force his terminator armour to move through sheer stubborn strength and willpower.