Select the rows or columns around the hidden lines
Right-click the selected row or column headers
Click Unhide
Or go to the Home tab
Click Format in the Cells group
Choose Hide & Unhide
Select Unhide Rows or Unhide Columns
To unhide all lines in a sheet, press Ctrl+A
Then use Home > Format > Hide & Unhide > Unhide Rows or Unhide Columns
If gridlines are hidden, go to the View tab
Check the Gridlines box
If hidden rows or columns are not visible, adjust their height or width by dragging the borders