If you have shadow-copying enabled on your Win7 installation, use Windows Explorer to navigate along the path of the bookmarks file that was shown in the "about Opera" entry: right-click the Explorer bookmarks file name > select Properties > select the Previous Versions tab > wait for the file search to take place (it can take a while) > if the system has saved previous copies of the file, select a copy from a known-good date and restore it. Not all users have their system settings enabled to make the automatic shadow copies, since the archive takes up disk space, so your ability to recover in this way depends on your Win7 settings. Also, do this sooner, rather than later, since each time you open Opera and it fiddles with the bookmarks file, the shadowing feature will save any altered version - and there's only so much space available in the archives... Win 7 deletes older shadow files to make space as newer ones are saved.
edited to add: If you dig out of the hole you're in, make a safe copy somewhere of that bookmarks file! And keep it updated from time to time. Consider this a wake-up call for creating a decent file backup strategy for your personal data files (eMails, bookmarks/notes, photos, etc, etc), then use that strategy thereafter and keep the backups reasonably refreshed (sometimes very old file backups aren't worth much more than no files). It's how I had to learn... 😉