Technicians believe they have identified the major technical issue which stopped the whole of Greater Manchester’s tram network on Sunday.
The Metrolink failure, which is not being linked to a cyber-attack, was discovered at about 15:50 BST.
The system automatically shut down following a communications fault, Transport for Greater Manchester said.
The fault was fixed shortly after midnight and trams are now set to operate normally, said a spokesman.
Transport for Greater Manchester said it was “not an issue Metrolink has experienced previously”.
It was caused by “a component attached to the Metrolink control network sending out spurious communications, which resulted in the safety system correctly shutting down”.
Transport for Greater Manchester said it runs a closed IT network with high levels of security and there was no indication of any form of hacking or cyber attack which would also result in shutting down the system.
Head of Metrolink, Danny Vaughan, said: “I’d like to apologise once again for the disruption caused on Sunday.
“We will conduct a review to look at what lessons can be learnt to avoid this type of issue occurring again.”