Thousands of homes and businesses in Burgess Hill were left without power for three hours due to repair work.
UK Power Networks said it had stopped supply to customers at around 10:00 GMT after it was informed of the fault.
The company said the work took longer than anticipated, with power restored at around 13:00 GMT.
A spokeswoman said: “We understand how difficult it is being without power, especially in colder weather.”
Barry Collins, from Burgess Hill, told BBC Radio Sussex: “My eldest daughter was at Haywards Heath College and it closed because their fire alarm system wasn’t working, so she was sent home.”
Several other schools had to close, including Gattons Infant School, Manor Field Primary School, St Wilfrid Primary and Sheddingdean Community, while traffic lights in the center of Burgess Hill were also affected.
American Express closed its office, and the Silverdale GP practice closed both its sites and had to direct patients requiring urgent treatment to the practice in Lewis or Crawley, or to the NHS 111 service.
James Seaton of Burgess Hill also said he was unable to work because of the power cut.
“I thought I’d make the most of my time and go and put some fuel in my car, but when I got to Tesco they were closed,” he told the BBC.
“I only had enough petrol to drive four miles, so I’m driving on fumes now.”
West Sussex County Council announced on Twitter that a planning meeting due for Thursday afternoon would also have to be rescheduled.
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