There are two possibilities –
1. Those rows (columns) are hidden.
2. Rows (columns) width is very small.
To uncover them –
1. Press CTRL+A to select entire sheet.
2. Double click on white line joining two numbers for row (two column alphabets in case of columns) and all these rows (columns) will be uncovered.
Question – I want to uncover only selected rows (columns)
Answer – Just select rows (columns) in such a way that one row (column) before those rows (columns) and one after are selected. Double click and only selected rows (columns) will be uncovered.