And they lived happily ever after. The End.
...Only it wasn't. The story ended, but life continued.
At first, things were good. They were young and in love and had the whole world at their feet. Eventually though, Prince became King, and the stresses and responsibilities drove a wedge between them that even wealth and privilege couldn't overcome.
Many unhappy years passed until their heir ascended the throne and they slowly started trying to get to know each other again.
These later years were better - not Happily Ever After again, but at least, perhaps, a new Once Upon a Time.