A Mass Ice Meltdown
The Sun released all of its solar flares at once. All of the polar ice caps have started melting. Every human scrambles to find higher ground. Quickly water, ancient deaseses brought back to the world, and radioactive waste from the old wars flowed into the newly created waterways. The oil companies drilled so deep in the Earth that there was no fossil fuels left. People turned to pagan religions, and the survivors of The aftermath started reforming the old ways. It's a new middle ages. Plants grew over the old oil companies drilling equipment, and the animal population was restored. Eventually after a couple thousand years Earth was restored to its natural beauty.