The History is on the left of the Develop section under Presets and Snapshots so you can migrate back through what you did. You can also go back to any snapshots you may have. If those sections aren't visible, right click in the left panel and you can re-enable them.
The other way - if you want to undo all the changes you have made to the image - is in Develop mode, there is a "Reset" button on the bottom right hand side of the window, next to the "Previous" button.