Without DNA ligase, the small pieces of the lagging strand would not be attached, so the lagging strand would be lost, and one of the new double helices would not be formed.
Work Step by Step
To answer, consider the role of ligase--to tie or bind together pieces of DNA. Then see that it is involved in the synthesis of the lagging strand. Then imagine what would happen to DNA replication without the tying together of the pieces of the lagging strand.