If you realize that you made an error in your project, you can roll-back to the previous state. You can do that by clicking the Undo command, available in the command Toolbar:
A quicker way to get the same result, is to use the keyboard shortcut Ctrl+Z.
There are several Undo levels available.
If, on the other hand, you realize that the Undo was not necessary, you can get back to the original state of the project by clicking the command Redo:
A quicker way is to use the keyboard shortcut Ctrl+Y.
Undo & Redo commands refer to operations that are stored in the temporary memory, hence they are not available as soon as you load a project.