To go back to square one means to have to begin from the start and redo something all over again.
1. The prototype of the chair did not work out so the designer had to go back to square one.
2. The badminton coach realized that his wards were not yet ready for the tournament so he would have to go back to square one in training tactics.
3. The psychiatrist would have to go back to square one to properly understand his patient's problem.