Why is it called "Divide by Zero"? Because Okabe tried to reach a world line that doesn’t exist (Steins Gate) without the necessary conditions. In computational terms, his emotional state divided by zero hope = undefined behavior. The universe crashed into a loop.

To understand Missing Link , you must recall Episode 23 of the original Steins;Gate :

Suzuha delivers the cold facts:

Okabe returns to August 21, 2010, defeated. Suzuha tells him that WWIII has already begun in the future. The time machine departs without him. Okabe then deletes the first D-mail from SERN's database, returning to the Beta line—but Kurisu remains dead.