«Volver a empezar desde cero»
To go back to square one means to start something out again. To return to the starting point so as to give something a new beginning.
When we play board games, we start from square one and the purpose of most games is to go advancing to the final box so as to become the winner. Some games include punishments in which the players have to go back to the starting line. A similar expression is to “Go back to the drawing board” as some architects would say. The intention is to repair a flaw, mistake or something that was incorrectly done.
My girlfriend told me that she wanted to quit our relationship. I love her so much and promised her that I would do my best to regain her love and trust. I proposed her to go back to square one and to start building a strong relationship from zero.