Some 80 NHS trusts across England have been hit by a "serious interruption" to their computer systems.
NHS Connecting for Health (CFH), the agency responsible for health service IT, said services provided by CSC Alliance for the NHS in the North West and West Midlands had been affected since 10:00 (BST) on 30 July.
It said the incident was caused by storage area network equipment failure which has also hit several other organisations using the affected CSC Maidstone data centre.
CFH said "technical issues" following power system interruptions mean data held on computers in the central data centre for the region could not be accessed.
A statement from CFH said: "The nature of the incident meant service could not immediately be provided by the back-up systems, also provided by CSC Alliance. No data has been lost."
But it said experts from CSC and its sub-contractor Hitachi have been working round the clock to restore access to data.
In the meantime the 72 Primary Care and eight Acute Trusts affected — all of which have had new administrative computer systems installed by CSC — are continuing to provide normal service by operating manual contingency systems.
CSC could not be reached for comment at the time of writing.