Most modifications made in GIMP can be undone. To view a history of modifications, use the undo dialog included in the default window layout or open one from the image window menu with Windows › Dockable Dialogs › Undo History.
The dialog shows a base image and a series of editing changes that can be undone. Use the buttons to undo and redo changes. In this way, you can often work back to the base image.
You can also undo and redo changes using Undo and Redo from the Edit menu. Alternatively, use the shortcuts Ctrl–Z and Ctrl–Y.