Goto the URL opera://bookmarks and look in the trash. If you see them there, you can hover over each one and choose "undo delete". Or, you can select one (or all), cut and paste them back in the "Speed dial" folder.
If you're syncing bookmarks with your phone, look at opera://bookmarks again for an "Other speed dials" folder to see the speeds from your phone. You can then select, copy and paste them into the "Speed dial" folder.
Besides those things, you'd need a backup.