Hi,

Electrochemical cell is a device used for production of electricity from energy released during spontaneous chemical reaction. If external potential applied becomes greater than Ecell of electrochemical cell, then the reaction gets reversed. It starts acting as an electrolytic cell and vice-versa.

When the opposing voltage greater than the cell potential is applied, the cell reaction stops altogether and no current flows through the cell. This is because there is no potential difference across the cell.
Further increase in the external potential now again starts the reaction but in the opposite direction. The cell now starts functioning as an electrolytic cell. In this case, electrical energy is used to carry out non-spontaneous reaction.

Hope this helps.