Lincolnshire County Council has suffered a suspected security breach on systems holding personal information such as medical records, addresses and bank details.
Emails were at first not impacted by the breach but have now been affected according to reports claiming to be from inside the council.
Council staff were ordered to close their computers and turn the power off.
At this stage, it is unclear who is behind the incident.
Judith Hetherington Smith, Chief Information Officer at Lincolnshire County Council, said: “We have a suspected security incident caused by malware.
“We closed down our systems very quickly to protect the data and are investigating the cause but at this stage have found no evidence of any breach.”
A spokesperson for Lincolnshire County Council said: “We have identified signs of a security incident and as a precautionary measure have suspended IT use until the extent of it is clear.
“Our customer service centre is still in operation but is running a limited service.
“Only call us on 01522 552222 if it is absolutely necessary.”