This sounds like a sync overwrite issue, which unfortunately can happen if a fresh install pushes empty data back to the server. Once that happens, Opera Sync usually replaces the older data, so recovery from their servers alone is difficult.
Before assuming it’s gone, try a few things:

1. Check if your old data still exists locally. Go to your previous user folder backup (if you have one) and look for Opera profile data, especially the “History” and “Sessions” files. If you formatted but kept a backup or used something like Windows.old, you might still recover it.
2. Try logging into sync from another device (phone, old laptop, etc.) that might still have the old data cached. If it hasn’t synced yet, you could export bookmarks or session info from there.
3. Contact Opera support immediately. In some cases, they may have limited server-side snapshots, but timing is critical.
4. Avoid further syncing actions for now, as they can overwrite any remaining traces.
   For the future, it’s safer to keep periodic manual backups of your Opera profile folder instead of relying only on sync, since sync is designed for mirroring—not version recovery.