The attackers had demanded €160,000 (£138,000; $183,000) in bitcoin, but a national decision was taken not to pay.
At hospitals still offline, IT teams raced to restore systems from backups.
Most had relatively recent copies of their data – a key lesson. Regular backups allow organisations to recover more quickly.
Within five days, most hospitals were back online and operating close to normal, with no reported deaths or serious harm to patients.
It would take weeks longer to input all the new information recorded on paper during the outage. Some data was lost forever.
Police are not commenting on their investigation into who was behind the attack.
However, last year a ransomware gang linked to BackMyData had its website taken down in an international operation.
Four Russians were arrested outside Russia, whose authorities do not co-operate with Western law enforcement.
Cimpean said the attack could have happened anywhere.
“The more technology you have, the more digitised you are, the greater the risk,” he said.
Last year the UK’s NHS health service confirmed a hack on a blood testing company that affected around a dozen medical centres in London contributed to a patient’s death.
It was the first case of a death officially linked to a cyber-attack.
Around the same time, Change Healthcare in the US was hacked, leading to widespread disruption. The company paid a $22m (£16m) ransom to hackers.
Hackers also caused chaos later in the year with an attack on another US healthcare provider called Ascension.
Alina Bîzgă from Bucharest-based cyber-security firm Bitdefender says attacks on hospitals are attractive to criminals who try to cause chaos for money.
“Hospitals handle critical services, and the criminals think that the more disruption that can be caused, the more likely they are to get paid a ransom,” she said.
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