The little boy lost in the lonely fen,
Led by the wand'ring light,
Began to cry; but God, ever nigh,
Appear'd like his father, in white.
He kissèd the child, and by the hand led,
The ‘wandr'ing light’ is a reference to will-o-the-wisp, or marsh gas produced naturally in marshy countryside from burning sulphur fumes. It can lure travellers into danger and is usually malevolent. 
  In Blake’s poem, it’s probably symbolic of whatever has distracted the person from following divine will. 

The Little Boy Found Songs of Innocence