I am putting together a business case model that requires a number of
charts/graphs. A few days ago the charts, which I had embedded within the
relevant tabs, were no longer visible when clicking on the relevant tab.
They appear as grey rectangles until clicked, when the chart then becomes
visible. I have tried deleting and creating new charts but the same problem
occurs.

What can I do to stop this from happening. I do not wish to recreate the
workbook as it took weeks to create.