Lost Calendar Notes on iPhone 15 Plus

I have an iPhone 15 Plus, and suddenly, two days ago (without performing any software update), all the notes I had in my Calendar were deleted and now none of them appear.

I have tried all the steps recommended by Apple, I restarted the device, I purchased additional iCloud storage thinking the notes might have been deleted due to lack of space, and I even installed the most recent software update, but nothing worked.

Could you please advise me on what I should do to recover my calendar notes?