Currys PC World has apologised after a website glitch wiped hundreds of pounds off gift cards and left Black Friday bargain hunters without their shopping.
The company is still assessing how many customers may have been affected.
But consumer website MoneySavingExpert.com says some customers saw hundreds of pounds vanish.
The retailer said the volume of shoppers seeking deals on Black Friday led to the problem.
“Due to the unprecedented volume of customers shopping online with Currys PC World this Black Friday, our website experienced a temporary outage,” the company said in an email.
“Unfortunately transactions being made by gift cards were affected. We will ensure that every customer affected by this issue has funds put back onto their gift cards in full, as soon as possible.
“We apologise for this issue and understand that customers will be disappointed they weren’t able to secure their products.”
Martin Lewis, founder of MoneySavingExpert.com, said the company should honour the deal prices of goods customers were trying to buy.
Currys PC World said it had seen home delivery orders rise by 59% and click-and-collect soar by 233%, with six orders taking place every second during the sales event.
Apple AirPods and the Nintendo Switch games console were the most popular items, it said.