I have the same problem but check this out. 3 weeks after they disappeared from my iPad they show up in my Yahoo email account. Go figure. Strange thing is that was the first place I looked when I lost them. Why it takes 3 weeks to get from my iPad to my email account is but another mystery but I am glad they finally showed up.
You might try checking under SETTINGS, then under NOTES to see if you have an account listed there, usually either an email account or an iCloud account. If so odds are your notes might be in one of these other places. But you might have to check for some weeks......