My son was having this issue and we tried all of the stuff recommended above to no avail but ended up resetting his phone and downloading his last back up from the cloud (thankfully it backed up last night) and the notes came back just fine. Highly recommend going this route if you have a recent backup from before they disappeared.
The method above (checking outlook through safari) was just posted after we got his notes back and moved from Hotmail to the icloud, and we tried it just to see and that worked as well. For that method you go to outlook and look at the area where you select a mailbox (inbox, sent, etc.) and there should be a section that says notes.
I just wanted to jump on here and confirm what Christy310 said about resetting her son's phone.
I lost my iPhone Notes (that were stored under my Hotmail setting in my iPhone) this morning, 10.31.24.
I went to my local Apple Store this afternoon and they said there was nothing they could do.
I came home and jumped online and found this thread; then I saw Christy's reply and tried it.
I wiped my phone and reset it to the backup from 5 days ago and all my Notes (304 of them!) came back just like she said.
Thank you so much, Christy!!!
Now I just have to move them all to my iCloud.
Ugh...