I’ve lost most of notes on my iPhone after the update yesterday. Where can I retrieve?

Did you also check the Recently Deleted folder in iCloud?

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You can recover and edit notes you have deleted in the last 30 days on iCloud.com or on your devices that are signed in with the same Apple ID. However, you cannot recover notes you have permanently removed.

1. In Notes on iCloud.com, select Recently Deleted in the folder list on the left.
2. If you do not see Recently Deleted, you do not have any notes in that folder, and there is nothing to be recovered.
3. Select a note, then click Recover in the Notes toolbar.
4. The note moves to the Notes folder.
5. If you wish to move a recovered note directly to a folder you created, drag the note from the Recently Deleted folder to another folder.

Note: If you delete a shared note that you do not own, the note does not move to your Recently Deleted folder. See Delete shared notes on iCloud.com.

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copied from: Delete and recover notes on iCloud.com - Apple Support

There are a few more things mentioned in this article:

If you're missing notes on your iPhone, iPad, or iPod touch - Apple Support

But, if you can't find the notes that you have synced with iCloud after that, your notes remain lost.

I have the same problem, most of the notes disappeared after I installed the newest Apple update. I called Apple support, they told me, many people have the same problem, and Apple technicians are working on fixing it.

Meanwhile, I was able to locate all my notes inside Yahoo mail, it suddenly created new Notes folder, and all the notes are there. I don't know how to retrieve them back into Notes on my iPhone, but at least I have access to them.