Select the worksheet area where the hidden cells are located
On the Home tab, click Format
Click Hide & Unhide
To unhide rows: select Unhide Rows
To unhide columns: select Unhide Columns
If rows/columns are not unhidden: select the rows/columns around the hidden area (including the row/column headers on both sides)
Repeat: Home > Format > Hide & Unhide > Unhide Rows/Unhide Columns
If cells are hidden due to filtering: go to Data > Clear (or toggle off) Filter
If cells are hidden due to workbook protection: review the sheet/workbook for protection and unprotect the sheet/workbook if allowed (Review > Unprotect Sheet)