I use a large spreadsheet about 45 columns and 300 rows. Something is causing the row heights of blocks of rows to be set at zero, which basically hides the rows. The only way you know they are there is the small double line in the row number column. I have tried to "unhide the rows, have tried to highlight the row above and the row below and set the row height to a larger number - nothing works. The only way I have found to fix the problem is position the cursor on the bottom double line and pull the row down larger. This works but is time consuming if 100-200 rows are hidden. I have fixed it several times, and later when I open the workbook, it has happened again.