If you are having trouble selecting the now invisible bar try the
following. Click on any of the bars that are visible in that series.
Then use the right arrow cursor key to cycle through the bars in that
series. Once the invisible bar is selected use CTRL+1 to display the
Format dialog.
If none of the bars are available for selection in that series just
select the chart and use the up arrow cursor key to cycle through the
major elements of the chart.