Power was lost across Ipswich Hospital amid a national power failure after the back-up generator failed to work.
The site was hit by a 30-minute power cut and staff are looking into why the back-up generators did not kick in.
People at the hospital said "sirens went off" as power was lost.
A spokeswoman for Ipswich Hospital said it was "too early" to say whether there was a fault but assured staff "helped keep patients safe" during the power outage.
Managers will be investigating the cause of the power problems at the hospital, she said.
Power failures have been reported across large areas of the UK, affecting homes and transport networks.
Vikki Irwin, a journalist with BBC Radio Suffolk, said her mother was at Ipswich Hospital at about 17:00 BST on a hydraulic lift being moved up to a scanner when the power cut happened.
"My mum was being lifted up and was near the top when all these sirens went off in the hospital and the lift stopped," she said.
"They said the power had gone out across the site. Lots of parts of the hospital were completely dark. We had to get my mum down manually."
Ms Irwin said the incident lasted at least 20 minutes.
'Never any danger'
Another woman at the hospital said she was with her friend in the cardiac unit when "the power just went off".
"It was only about two minutes. Everything came back really quickly," she said.
"People seemed calm. There was no panic."
A hospital employee told the BBC the power cut did not last long on the ward she was working on.
"It was just a few seconds, definitely no more than 10," she said.
"There was never any danger."