This can happen when the display resolution changes for some reason and the notes go out of visible desktop area.
The display resolution can change during screen saver, sleep mode etc.
In such cases, you can go to the main
Notezilla menu from the taskbar icon and choose 'Arrange notes' option.
In Settings/Preferences of
Notezilla , under General tab (towards the end) there is an option you can choose to configure what action
Notezilla must take when display resolution changes. You can modify that setting accordingly.