Answer:
The boy has chased after something unreal, leaving behind the reality and solidity of human relationships (the only really real things), and so he ends up lost, emotionally and spiritually. Pursuing something unreal instead of something real and getting lost, could be an allegory for the spiritual life gone wrong, whether we have chased after riches or power or luxury.
Blake was sharply aware of reality: in real life once a person is “lost” (whether to drugs or drink or other cravings and unrealities) then they often stay lost and never recover. But these poems are from his Songs of Innocence, which, to some extent, portray his ideals, his deepest spiritual convictions. So here, in the companion poem, “The Little Boy Found”, the boy does not stay lost: he is rescued by God who appears in the form of his father. Complex theological (religious) ideas are at work here. Blake did not believe that God was some great giant or superhuman old man up in the sky.