How do I retrieve all my notes that I accidentally deleted?

Presuming you're using the included Notes app, have you looked in the Recently Deleted folder?

And a friendly tip: have a bit of patience when you ask a question. This isn't live chat and Apple Support does not participate in these forums. We're all just fellow users here, helping as we have time and knowledge to share, so it may take more than the one minute you waited before anyone even sees your question, much less has time to formulate a diagnosis and post a suggestion.

Regards.

You have 30 days to retrieve any recently deleted notes from the Recently Deleted Notes folder. After that, those notes will be permanently deleted and cannot be recovered.

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