More than 26,000 homes and businesses in North Yorkshire were hit by a major power failure on Thursday night.
The problem began when electricity supplies were disrupted at Catterick Garrison, affecting 9,000 customers, said power company CE Electric.
An attempt to switch to an alternative feed then knocked out sub-stations supplying 17,000 homes in Bedale, Wensleydale, Thirsk and Northallerton.
Engineers finally restored all supplies after a blackout lasting four hours.
A CE Electric spokesman said: "We want to apologise to all the customers affected and would assure them that our engineers worked flat out to get supplies back."
The company said it was carrying out an investigation on Friday to find out what caused the original power cut at Catterick.