Notes have disappeared
When people report that their notes have disappeared, it seems that they usually mean that the Notes Pane, the area that appears beneath your slide when you're working in Normal view, isn't visible for some reason.
This may happen for several reasons.
PowerPoint is in Slide view
Before PowerPoint had Normal view (with slide thumbnails, the slide itself and notes text below) there was Slide view, a view that showed just the slide. There's no obvious way of making this appear; you can't get there from the View tab, for example, but it can happen.
Sometimes add-ins will inadvertently leave PowerPoint in Slide view.
Or you can deliberately go to Slide view by pressing SHIFT + CTRL while you click the Normal View icon at the right of the status bar, bottom of your PowerPoint window.
Honey, I shrunk the Notes!
For some reason, Windows 10 seems to cause some programs to forget their window size and placement settings ... including the size of the notes pane. It can shrink to nothing!
The fix: If you're in Normal view, hover your mouse over the thin line at the bottom of the slide pane. The mouse pointer will become a cursor that will allow you to drag the line upwards, making the notes pane taller and revealing the notes within.
Alternatively, hover the cursor above the icons at the right of the status bar at the of the PPT screen until you find one whose tooltip is Normal. Click this to alternate between Outline and Slide Thumbnail view in the pane on the left of the screen.
AND it'll restore a shrunken Notes pane to a reasonable size.
There's always Notes Page view
You can always choose View | Notes Page.
Regardless of whether the Notes pane is showing in Normal view, the notes, if there are any, will be visible there.