Which Power Rangers series followed Mighty Morphin in the continuity?

Power Rangers Zeo followed Mighty Morphin Power Rangers in the franchise's main television continuity.

Zeo begins after the Command Center is destroyed and the Rangers discover that the Zeo Crystal has survived. The former Mighty Morphin team adopts a new power source, new suits, and a new series identity while continuing the story of Tommy Oliver, Adam Park, Rocky DeSantos, and Katherine Hillard. Tanya Sloan joins as the Yellow Zeo Ranger, while Billy Cranston becomes the team's technical adviser.

The main new threat is the Machine Empire, led by King Mondo and Queen Machina. Zeo also introduces Trey of Triforia as the original Gold Ranger; when Trey must recover his powers, Jason Lee Scott temporarily becomes the Gold Ranger.

The main point of confusion is Mighty Morphin Alien Rangers. That short miniseries follows the third season of Mighty Morphin and acts as a bridge into Zeo, including the Rangers' quest to restore their ages and recover the Zeo Crystal. However, Zeo is the next full Power Rangers series and the canonical continuation of the main team under a new title.