A unit that makes a failed charge has started towards the enemy, but is unable to cover the distance. The warriors in the unit simply lose impetus. If a unit makes a failed charge it moves directly towards the target a distance equal to the result of the Charge roll, wheeling as required.
Fig 121.1 - Unit A has declared a charge against unit B. Unit A's Movement characteristic is 4" and a Charge roll of 1 and 5 has been made, for a total charge range of 9". Alas, unit B is 10" away, so the charge has failed. Unit A must now move 5" (the highest result of the two dice rolled).