You've probably lost them.
If you go Menu>Help>About Opera it will show you the path to where Opera keeps the Bookmarks.adr file open it with notepad it will probably confirm that you've lost them.
Set up Opera Link & backup your bookmarks & notes to stop it happening again.
Since using Opera Link I've never lost my bookmarks upgrading my Computer.
Oh yeah maybe you are new to Opera, try Ctrl B or look in the menu under Bookmarks . Apologies i don't mean to insult you with very basic stuff but some of my friends would consider Ctrl B advanced.