If this happens usually your y-axis has been formatted wrong so you are not able to see it. First you have to mark the chart, then you go on opening the tab under ‘chart tools’ or ‘layout. There you will find ‘Current Selection’. Select the vertical axis in this menu. After this, you simply click on ‘Format Selection’ and choose ‘Format Axis’ → ‘Adapt Axis Options’. This will make your y-axis visible again.