Doctors and hospital staff were unable to access patient details due to a widespread failure in computer systems affecting the Welsh NHS on Wednesday.
Consultants and GPs said they could not get blood and X-Ray results because their internal portal was down.
The Welsh Government has since confirmed that all systems are now back up and running.
But it warned "there may be a backlog affecting some areas".
The National Cyber Security Centre said the problems were caused by technical issues and were not the result of a cyber attack.
The problems did not lead to any data security issues, according to the Welsh Government's department of health and social services
The NHS Wales Informatics Service have confirmed that all systems are now back although there may be a backlog affecting some areas. They have assured us that there were no data security issues. 1/2— DHSSWales (@DHSSwales) January 24, 2018
The British Medical Association (BMA) in Wales, which represents doctors, said it understood at least half of GP services had been affected.
One GP called the situation "chaos" and said: "I can't do anything. I need this system for everything."
Earlier, a warning on doctors' computers read that "multiple services are affected, especially national services".
Bangor family GP Dr Catrin Elis Williams described the situation as "very frustrating and rather dangerous".
Her computer system went down about lunchtime on Wednesday.
"Unfortunately it's caused us quite a lot of problems with trying to get patients' results, past medical histories and letters from hospitals," she said.
"We have seen a few people we can't help because we can't get that information."
The fact the system went down also meant staff could not get phone numbers of patients to contact them and cancel appointments.
Other problems included the fact notes from appointments could not be typed up and saved, meaning there will be a backlog.
"The system has gone down before but only for short periods," she added.
Philip Young, 48, from Caerphilly went to the Royal Gwent Hospital in Newport on Wednesday to find out the results of a biopsy he had taken from a mass on his vocal chords two weeks before Christmas.
But he was told the results were not available due to the systems being down and that he should return in a month.
His ex-partner Emma Noble said it was "very concerning" and she was "annoyed for him".
"To have to wait another month is completely unacceptable, we could be talking about cancer here," she said.
"It's not just something you can leave."
Ms Noble said she had called the Aneurin Bevan University Health Board complaints line, but it was inundated and she would have to call back in the morning.
The assembly's opposition health spokesman Angela Burns said the problems were "incredibly concerning, especially in the aftermath of last year's ransomware attack on the NHS."
"The focus must now be on supporting these centres to manage this incident swiftly so that patient data is secure and that care is not adversely affected," said the Conservative AM for Carmarthen West and South Pembrokeshire.
"This is under investigation by the NHS Wales Informatics Service and is being dealt with as a priority," a Welsh Government spokesman added.
Powys NHS IT department said: "There is currently a major incident affecting a number of nationally hosted systems across NHS Wales.
"The issue appears related to national firewalls and is being currently investigated.'"
Dr Alan Woodall, chairman of GP Survival UK who works from a surgery in Montgomeryshire, said: "Myself and colleagues that work in primary care cannot access electronic patient records which is severely inhibiting our ability to provide patient care."
Another doctor, working from a GP Surgery in the Abertawe Bro Morgannwg health board area that covers Swansea and Bridgend, described it as "chaos", adding: "I can't do anything. I need this system for everything."
A consultant at Morriston Hospital in Swansea told the BBC he could not access blood and X-Ray results on his computer because their internal intranet portal is not working.
A senior registrar at the Royal Glamorgan Hospital in Llantrisant said the problems started mid-morning on Wednesday.