When you work with a worksheet, Excel records every change you make. For example changes in cells, cell and text formatting, cutting, copying, pasting, editing, etc.
Excel can successfully save up to 100 changes, without slowing down the computer.
You can control these changes by using two commands located in the Quick Access Toolbar: Undo and Redo.
If you want to go back to the particular state, click the triangle next to the Undo icon, then choose a position you want to go back to. The changes are described, so you can go back to the exact state, without guessing.
The Redo icon becomes active when you go back at least one position. With Redo as well as with Undo you can click the triangle (that is located in the same button) and select an item from the list.
Instead of clicking the arrows, you can also use the Undo (Ctrl + Z) and Redo (Ctrl + Y) keyboard shortcuts.
Some actions cannot be undone. One of them is removing a worksheet from the workbook.