About 55,000 homes around Lancaster are without power and could be cut off for days after an electricity substation was flooded during Storm Desmond.
Electricity North West said flood defences upgraded five years ago to cope with a "one-in-a-100 years flood" were breached on Saturday night.
All but emergency operations have been cancelled at the Royal Lancaster Infirmary on Monday.
And at least 10 schools in Lancashire will remain closed due to flooding.
Engineers were only able to access the substation in Caton Road on Sunday after it had been "fully submerged". Operations director Mark Williamson said they were assessing the damage.
He said it was not know how long repairs would take but it "could be days" before power is restored.
Some students at Lancaster University were leaving the campus due to the power cut and problems with running water.
Lancashire County Council said the 10 schools known to be closed on Monday are:
- Appletree Nursery
- Carnforth High
- Great Wood Primary
- Lancaster Girls' Grammar
- Over Kellet Wilson's Endowed CE Primary
- Quernmore CE Voluntary Controlled Primary
- Scotforth St Pauls CE Primary and Nursery
- St Peter's CE Primary, Heysham
- The Cathedral Catholic Primary, Lancaster
- Trumacar Nursery and Community Primary, Higher Heysham
The University Hospitals of Morecambe Bay NHS Foundation Trust said it was cancelling all non-urgent operations and a number of clinics at the Royal Lancaster Infirmary including audiology, ear, nose and throat and maxillofacial clinics and breast screenings.
A trust spokesperson said: "We are trying, where possible, to contact those patients affected, but due to significant problems with phones in the Lancaster area, this is difficult.
"Therefore, we are asking for the public's help in passing on the message to any friends and family members that may have appointments tomorrow."
About 350 army personnel have been made available from 2nd Battalion Duke of Lancaster's Regiment, based in Weeton Barracks near Preston, to assist with the general flooding response from the early hours.
They arrived with two vehicles to help evacuate and warn residents in Carlisle - and sent a Chinook helicopter to move high-velocity pumping equipment needed at an electricity substation.
Lancashire Fire and Rescue Service said Storm Desmond has proved to be "a really difficult time for all emergency services".
Chief fire officer Chris Kenny said their efforts on Saturday night focussed on moving vulnerable people who were stranded where ambulances couldn't get through.
"Our focus then was trying to stop the power outage at the Lancaster substation," he said. "You're battling against rising water and it's relentless."
The county's firefighters dealt with more than 300 calls for help in Lancaster and surrounding areas as flooding hit the Cable Street, Water Street, Chapel Street, Damside Street and St George's Quay parts of the city centre last night.
Crews from all over the Lancaster and Morecambe district attended a range of situations including helping cars stuck in floodwater, affected electrics, flooding and road traffic collisions and two fires.
Marco Petagna, a senior forecaster at the Met Office, said parts of Cumbria and Lancashire have been hit with more than a month's worth of rain in just 24 hours.