The trouble with having a brain as large and as vast as Sherlock's is that sometimes it works against him. Not often, mind you, or else the world's only consulting detective wouldn't exist.
John has come to know Sherlock's moods quite well, the ups and downs, the exhilaration and the sulking; particularly his occasional overwhelm of boredom that soon becomes insufferable to both parties.
There are times, however, where Sherlock is simply… lost.
John privately refers to them as 'black moods,' dark and empty. A quote tugs at the back of John's mind: "If you gaze long enough into an abyss, the abyss will gaze back into you." He wonders if Sherlock is familiar with the sentiment, or if he'd perhaps deleted schools of philosophical thought along with the solar system.
The vacancy of Sherlock's expression is troubling, and sometimes John fears that whatever intrinsic matter that makes up Sherlock will never resurface, that one day he will remain adrift in his mind palace, aimlessly wandering its multitude of intricate halls. It's at times like these John feels most helpless, but there's a lone thought that gets him through each time it happens.
The surety he feels that no matter how far Sherlock may wander, however long it takes, John will always be there to lead him out of the black.