IT experts are working to fix NHS computers after they were targeted by hackers in a cyber attack on Friday.
Hacking is when someone breaks into a computer system illegally. A cyber attack is when computer systems are deliberately damaged or disrupted by people.
The attack meant some hospitals and doctors in England and Scotland were not able to use their computers and some appointments had to be cancelled.
The NHS says patients needing urgent treatment should access emergency services as normal.
As well as the NHS, it's thought the cyber attack has affected companies in about 100 other countries.
The NHS has not been affected in Wales and Northern Ireland.
IT experts are working really hard to fix the problem so NHS services can get back to normal as soon as possible.
It is not yet known who carried out the attack but there will be investigations into exactly what happened.