I found links to my Missing Pinboards on this Opera web page: https://www.sync.opera.com/pinboards
Or you can find your missing Pinboards via this link too: https://www.sync.opera.com/
I restored my missing Pinboards by means of the procedure described below.
You must be signed in to your Opera account with Synchronization enabled. Go to Opera Settings - Synchronization - Advanced sync settings then checkmark Sync Everything
I think the latest version of CCleaner Pro wiped out my Opera Pinboards (from my PC cache). So, after restoring my Operal Pinboards, I went into CCLeaner Options - Cookies and found pinboard.opera.com and whitelisted that cookie.