"Power unit No 6 was unloaded and disconnected from the grid" because of a fire that was "triggered because of shelling", state-run company Energoatom said on Monday.
Ukrainian President Volodymyr Zelenskyy said on Monday that the new cut-off had placed the plant for a second time "a step away from a radiation catastrophe".
Mr Zelenskyy said Russian shelling was responsible.
"Again — already for the second time — because of Russian provocation, the Zaporizhzhia station was placed one step away from a radiation catastrophe," he said in his nightly video message.
"The shelling of the territory means that the terrorist state does not care what the IAEA will say. It is not concerned about what the international community will decide.
Ukraine's Energy Minister, German Galushchenko, later said: "The world is once again on the brink of a nuclear disaster.
"The de-occupation of the ZNPP and the creation of a demilitarised zone around it is the only way to ensure nuclear safety."
This was the last working reactor out of six, after shelling disconnected reactor No 5 on Saturday, the International Atomic Energy Agency (IAEA) said.
"After the ZNPP connection to its last remaining operational 750 kilovolt line was lost late on Friday, the 330kV reserve line had been used to deliver electricity from the ZNPP to the grid," the UN nuclear agency said on Monday.
"Ukraine informed IAEA that this back-up line will be reconnected once the fire has been extinguished.
"A secure off-site power supply from the grid and back-up power supply systems are essential for ensuring nuclear safety."
Zaporizhzhia nuclear power plant attacked - in pictures
EU high representative Josep Borrell, speaking alongside Ukraine's Prime Minister Denys Shmyhal in Brussels, said the news was "worrisome".
Mr Borrell said the "nuclear gamble has to stop" and accused Russia of "reckless behaviour, disdain for international law, basic principles of nuclear safety".
Last week, a 14-strong team from the IAEA visited Zaporizhzhia, with the UN nuclear watchdog's chief Rafael Grossi saying the site had been damaged in fighting.
Mr Grossi and part of his team left the site on Thursday, but several members of the mission stayed at the plant to conduct more in-depth analysis.
Out of the six experts who stayed, four left on Monday and the remaining two are expected to stay in the power plant "on a permanent basis", Energoatom said on Monday.
IAEA issues warning over military activity near Zaporizhzhia nuclear plant - video
Mr Grossi will on Tuesday issue a report about nuclear safety in Ukraine that will include the mission's findings, the IAEA said, and he will brief the UN Security Council on the same day about the visit.
Ukraine was the scene of the world's worst nuclear disaster in 1986, when a reactor at the northern Chernobyl plant exploded and spewed radiation into the atmosphere.