After the country's 10 million people had been plunged into darkness overnight, the Caribbean island's national power grid came fully back online by 6:11pm on Tuesday (9:11am Wednesday AEDT).
However, officials said power shortages may continue because not enough electricity was being generated.
In addition to cutting off oil sales to Cuba, US President Donald Trump has escalated his rhetoric against the Communist-run island, saying on Monday he could do anything he wanted with the country.
A US State Department official blamed the Cuban government for the grid collapse, calling blackouts a "symptom of the failing regime's incompetence".
Cuban President Miguel Diaz-Canel fired back at Washington, criticising its "almost daily public threats against Cuba."
"They intend to and announce plans to take over the country, its resources, its properties, and even the very economy they seek to suffocate in order to force us to surrender," he wrote on social media, shortly after power returned nationwide.
Cuba has yet to say what caused Monday's nationwide grid failure, the first such collapse since the United States cut off Cuba's oil supply from Venezuela and threatened to slap tariffs on countries that ship fuel to the island nation.
By midday on Tuesday, grid workers successfully fired up the Antonio Guiteras power plant, a decades-old behemoth that underpins the country's power grid.
Electricity generation, hampered by dire fuel shortages and antiquated power plants, is still far below what is necessary to cover demand, providing scarce relief for Cubans already exhausted from months of blackouts.
Most Cubans, including those in the capital Havana, were seeing 16 or more hours of blackout daily even before the latest grid collapse.
Cuba has received only two small vessels carrying oil imports this year, according to LSEG ship tracking data seen by Reuters on Monday.
On Tuesday, a Hong Kong-flagged tanker that could be carrying fuel to Cuba resumed navigation after suspending its course weeks ago in the Atlantic Ocean, according to LSEG ship tracking data.
Cuba and the United States have opened talks aimed at defusing the crisis, among the most acute since 1959, when Fidel Castro forced a US ally from power on the island.
Neither side has provided details of the ongoing negotiations, although Trump has portrayed Cuba as desperate to make a deal.
Washington would be doing "something with Cuba" very soon, he said in comments to reporters in the Oval Office on Tuesday.
