Argentine President Mauricio Macri has promised a full investigation.
Argentine media said the power cut occurred shortly after 07:00 (11:00 BST), causing trains to be halted and failures with traffic signalling.
The blackout was prompted by a failure in an electrical grid that serves both Argentina and Uruguay.
The outage occurred as people in Argentina were preparing to go to the polls for local elections, delaying voting in several regional provinces.
Parts of Paraguay were also affected, a state energy company said.