Notes in my "on my iPhone" folder just disappeared but they are not in the recently deleted folder

Hello eliju!  

Thanks for reaching out to the Apple Support Communities. From your message I understand that you’re no longer seeing your “On My iPhone” account. The first step will be to go to Settings > Notes and see if the option for “On My iPhone” account is turned on or off.  This is the setting which allows you to store notes on your iPhone. Once you have it turned on you’ll want to decide which you want the default account to be, iCloud or “On My iPhone.” You can learn more about Notes from this article: Use Notes on your iPhone, iPad, and iPod touch. 

Cheers!  

Open the Notes app and tap the "<" icon at top-left corner to take you to "Folders". There you should see an "On My iPhone" category -- select that and you should see your notes.

If you've done this and still don't see your notes, notes that are stored on your device are part of your iOS backup, so in theory you could restore your iPhone and load your last backup, but if you backed up after you lost the notes then you're out of luck.