More than 150 properties in the town centre were left without electricity after a fault on an underground cable.
The area around the High Street was affected, along with the Tuesday Market Place and St Ann's Street.
Scores of cash machines were out of use after the outage, at around 1.30pm. Power was restored at 3.30pm.
Manager of the Corn Exchange Philip Bayfield said: 'Luckily we do not have a show this evening but we have had to turn customers away from our coffee shop and box office.
'We have contacted EDF and they have informed us this will probably not be fixed until 3pm or 4pm.
'It is an inconvenience but shows how much we rely on electricity.
'If it is not on later we are going to lose some stock such as ice cream etc.'
One Boots sales assistant said: 'This is our busiest time at lunchtime so we're losing a lot of customers.'
Brown & Co estate agents on Tuesday market place have tweeted: 'That's a good start to the week! #powercut #kingslynn :('
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