If you're running Windows and have its built in shadow copying enabled on the system and drive where the bookmarks file is located, navigate to that file's folder and locate the bookmarks file. Right click it, select Properties, and select the Previous Versions tab at the top of the panel. If there are previous versions stored in the shadow copy records, a list by date and location will appear after a sometimes long search time. You can select an appropriately dated version in order to replace the current version of the file. Do any file replacement/restoration with Opera turned off. Note that shadow copies are only created if the option has been enabled; also, the shadow copy space is limited, and very old files eventually may be removed to make space for new files, over time.