A Hertfordshire hospital declared a critical incident and told people not to attend after a technical issue with oxygen equipment.
Watford General hospital asked people to stay away following the incident, which they said did not pose any risk to patients.
West Hertfordshire Hospitals NHS Trust said the problem was resolved by 10.30pm on Saturday.
The trust had transferred patients to other hospitals in the area and tweeted telling people not to go to A&E until further notice even in an emergency.
Please DO NOT attend Watford General’s emergency department until further notice, even in an emergency. Go to your next nearest hospital with an emergency department. Anyone with non-urgent issues should seek advice via https://t.co/3rSnMp3o5g
— West Herts Hospitals (@WestHertsNHS) April 4, 2020
It advised people instead to visit other nearby hospitals or seek advice through the 111 helpline.
Deputy chief executive Helen Brown said: “We know that our NHS colleagues and the police are exceptionally busy at the moment, but this didn’t stop them stepping in to help.”
As of 5pm Thursday, 29 people had died at West Hertfordshire NHS Trust hospitals after being diagnosed with Covid-19.
The trust is responsible for Watford General as well as Hemel Hampstead and St Albans City hospitals.
Watch the latest videos from Yahoo UK