What does George Bush mean by "night fell on a different world"?

George W. Bush quoted the words you have cited* in an address to a joint session of Congress and the American people on September 20, 2001, nine days after the horrific acts of terrorism inflicted upon the country on September 11 of that year. A link to the full speech is included below this response, and the immediate context surrounding the quote reads as follows:

Americans have known the casualties of war—but not at the center of a great city on a peaceful morning. Americans have known surprise attacks—but never before on thousands of civilians. All of this was brought upon us in a single day—and night fell on a different world, a world where freedom itself is under attack.

By asserting that the world drastically changed during the course of one single day, the President calls attention to the extreme effects of the four coordinated terrorist attacks that unfolded in the US on the morning of September 11, 2001. A weekday that began unremarkably concluded with a death count of almost 3,000 innocent people, impacted the lives and well-being of countless others, and had long-lasting effects on New York City, the United States, and the entire world. In this speech, the President attributes the attacks to "a fringe form of Islamic terrorism" and introduces the al Qaeda terrorist organization, it's then-leader Osama bin Laden, and the Taliban regime of Afghanistan to the forefront of national consciousness. The term "war on terror" was popularized as a result of this speech, in which the President vows to "stop [terrorism], eliminate it, and destroy it where it grows." The quote notably and accurately describes a terrifying, exhausting day that forever impacted the world.
*When completing this assignment, note that although your original question names the President as "George Bush," it is important to cite his middle initial in order to distinguish between the two presidents bearing that name.
https://georgewbush-whitehouse.archives.gov/news/releases/2001/09/20010920-8.html