Currently, there is no “undo” tool in the Rich Content Editor. However, there are keyboard shortcuts you can use to undo the last action you completed.
Simply click CTRL-Z while in the Rich Content Editor and it will undo the last action you completed. You can also do this by clicking on ALT-F9, then clicking on the Edit drop-down list, and selecting “Undo”.
You may also be able to recover accidentally deleted Assignments, Discussions, Modules, Files, etc. by clicking the Home button of your course and type /undelete at the end of the course URL in the address bar.
Then click the Restore button next to any item you wish to restore to the course.