What phone and OS version? And did anything change, such as a system update?
The name "notepad free" looks like a third party app rather than a vendor one; that doesn't mean it wasn't pre-installed (plenty of vendors do this, often for a fee) though it's unusual to see it advertise that it's the free version. But I have seen third party apps included in the system software disappear with a system update: my Samsung tablet had a mass of such apps originally (some couldn't even be disabled) but with an android version update many of them went. As I wasn't using them I paid little attention, but that's why I ask about your phone and about updates.
One thing you could try if it's really gone: reinstall from the play store. If you had your phone set to back up apps data with Google and it recognises it as the same app there's a chance it will restore the notes. But I don't know how likely, so I'd make really sure it's gone (not just missing from a desktop) first.