I dreamt a dream!  What can it mean?
And that I was a maiden Queen
Guarded by an Angel mild
Witless woe was ne'er beguiled!
And I wept both night and day
And he wiped my tears away
And I wept both day and night
This is saying that foolish sadness (‘witless woe’) is never ‘beguiled’, the latter being a now rarely-used word meaning ‘entrance’ or ‘tempt’.  In short this ‘maiden Queen’ was receptive to the loving guardian ‘Angel’. 
 In Blake’s time innocence was a valued ideal. The angel is there to protect the maiden.
The Angel