As the clock struck midnight, two things happened: The federal government shut down, and the Affordable Care Act began open enrollment.
President Obama's landmark legislation had been the point of contention between Congressional Republicans and Democrats in the weeks (and even years, I suppose) leading up to the 2013 government shutdown. Though the House GOP held fast to their demands for a repeal or delay of the ACA in return for a continuing resolution on government spending, it was all for naught. The exchange marketplaces opened at the same exact time much of the rest of the government was shutting down.
Obamacare's launch was inevitable, as the main aspects of the ACA fall under "essential" government services, which remain operational during a government shutdown.