Excel workbooks keep breaking

This sometimes starts happening with workbooks with a long editing history. I would start with checking the range names (Formula tab, Name Manager). Anything there that you don't need you should get rid of. Also, on the Home tab, click the Cell Styles drop-down. Does it show many styles in the "Custom" category?

To resolve this it is advised to try to create a fresh workbook. Open a new blank file and insert as many worksheets as your old one has. Name them exactly the same. Now copy the used area from each tab in the original and do a Paste Special, Formulas into the associated tab of the new file.

Of course any special items need to be rebuilt, such as tables, charts, pivot tables etcetera.