Select the entire worksheet by clicking the Select All button in the top-left corner or pressing Ctrl+A twice
Right-click any row number and choose Unhide
Go to the Home tab
Click Format in the Cells group
Select Hide & Unhide
Click Unhide Rows
If needed, adjust row height by selecting the rows and choosing Row Height from the Format menu
If rows are still hidden, check for filters and clear them
If rows are grouped, expand the outline using the plus buttons or the Data tab options