Try to check if there is a back up of the Bookmarks file localized in | Opera Menu > About Opera > Paths > Profile |

Opera Bookmarks Back Up: File called | Boomarks.bak |

Close Opera, go to profile path with Windows Explorer, make a copy of both files Bookmarks and Bookmarks.bak (Just for Security)
Then rename Bookmarks to | Bookmarks - Failure | or whatever to know it's the file without your Bookmarks and Rename Bookmarks.bak to Bookmarks.

Open Opera with this file and check if your Bookmarks are there.

And / Or check if there is a Windows Back Up

Go to same path with Windows Explorer, rRght click on Bookmarks file > Properties > Previous Version

You may use sync using your Opera Forums User/Password to avoid future problems like this.