A Gloucester-based firm which hosts UK websites is being blamed for thousands of pounds in lost revenue after changing passwords without notice.
Businesses received an e-mail on Friday telling them Fasthosts had reset their passwords as a security measure after being hacked in October.
The move meant all those affected could not access their websites nor could any of their customers.
Fasthosts has since apologised to those affected.
In a statement the company said: "A number of Fasthosts customers had their control panel and FTP passwords automatically reset.
"Last month, Fasthosts wrote to all its customers to advise them that the company had discovered a network intrusion involving a Fasthosts server, and as a precautionary measure recommended that all customers update their control panel, FTP and e-mail passwords.
"As a result, Fasthosts has implemented automatic password changes to every customer that did not change their control panel and FTP passwords as was previously recommended.
"Fasthosts will also change all unchanged email passwords automatically 10 days from today."
New passwords were due to be posted to clients using Royal Mail.
"This is the busiest retail time of the year - how dare they keep us down for a whole weekend when people buying online will be most active," said e-businessman John Rudolf who runs website South West Mafia.
"Posting passwords is farcical. Since all business is usually done to this company via e-mail, mass e-mailing would be as, if not more, secure and incidentally almost instant," said Mr Rudolf.
"My site could be back online within a minute if I had the passwords - that is what is so frustrating."