The Ending was a catastrophic event that killed most of the world population.
Unknown at this moment. There are some who believe that Kleya caused it. It is unknown whether the catastrophe was directly linked to hacking of the Mechas.
Unknown at this moment. The Dude mentioned hacking the geomagnetic field. There were several volcanos, including a Super Volcano. During this time, the atmosphere became very dangerous, for some reason.
Most of the world's population was destroyed. Earth became very unstable. Earthquakes and solar flares became common. Cities became the gathering places for survivors. The sky no longer is blue. The stars are no longer visible. The poles have been moving. In addition, large volcanoes have been very frequent. Many large volcanoes dumped ash into the air.
However, the situation has been gradually getting better. According to D, the south pole has only moved a little while Kleya was in LiFe. There were only a few volcanoes during that time, as well, and none of them shot ash into the atmosphere.
Immediately after The Ending, there were Hacker Hunts similar to the historic Witch Hunts, killing anyone and everyone who knew programming. With very few people left who knew programming, much of the technologies fell into disrepair. The world became a pretty bleak place.
Originally, page 30 indicated that 2/3 of the world population but it has apparently been updated to state "most of Earth's population"