How to recover disappeared notes on my iPhone 8?

Yesterday it was fine, but when I opened it this morning all my notes are gone?? I rely very much on the notes app and I really need to get them back somehow. I tried rebooting my phone and it didn’t work. I don’t know what to do because I have stuff on there that I need to retrieve. When I open the notes app it just shows Folders and when I try to make a new note and save it, it doesn’t save. Can someone please help or give me advice because I really need the notes app.

[Re-Titled by Moderator]