- The gridlines are turned off. (Fix: View > Recheck box for gridlines).
- The gridlines are colored over, usually with white. (Fix: right-click on cell > Format Cells > Fill tab > Select No Color, or select white and then reselect no color)
- The borders are colored white. (Fix: right-click on cell > Format Cells > Border tab > change color to Automatic)
- The gridlines have been colored white. (Fix: click on File > Options > Advanced > Display options for this worksheet > change Gridline color to black)
Found these reasons from this handy page:
My particular fix came from the second reason. I've tried it on Excel 2010 and Excel 2013.