It was stopped as a safety measure after last week's attack on the Nova Kakhovka dam.
The structure's collapse sent a cascade of water downstream, wiping out towns and villages.
Five out of six reactors at Zaporizhzhia - occupied by Russian forces but staffed by local workers were already in a state of cold shutdown.
Ukraine's nuclear agency Energoatom said the possibility of there not being enough water to cool the remaining reactor following the dam's destruction meant it was forced to follow suit.
The process involves control rods being inserted into the reactor core to stop the nuclear fission reaction and generation of heat and pressure.
The news emerged as the UK donated a further £16million in humanitarian aid to help the 42,000 people directly affected by the dam breach, which is being blamed on Moscow.
Hundreds of Ukrainians have been rescued from rooftops in the days following the deliberate breaching of the Kakhovka dam, which flooded villages, fields and roads in the Kherson region.
Last night, the Foreign Office confirmed the cash will be given to organisations on the ground. The Red Cross will get £10million, £5million is going to the UN Office for the Coordination of Humanitarian Affairs and £1million to the International Organisation for Migration.
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