Well, what went down on Earth during the timeskip isn't going to be part of the story except as oblique references, but let's see...
There was the Eternal Crystal that someone made the centerpiece of a theme park which attracted some nerds from Eltria (this was later made into the Reflection/Detonation movies which exist in canon but the movie's depiction of the events is not canon),
Some rabbit-eared researcher almost caused an international crisis thanks to getting facehugged by an ancient knowledge repository loaded with information on powered exoskeletons complete with a standard complement of unit cores,
The castle everyone thought had been sealed inside of an eclipse by the combined efforts of several people including Gil Graham managed to become unsealed and was only put back with the assistance of a man they called Arikado,
A bunch of AI programmers accidentally tapped into another world at one point,
Nanoha's friends and family probably got sucked into something that would make future good movies I'm forgetting to mention here at least two more times...
No points for IDing each of the references.
Geez.
Earth's a pretty busy place, huh?