Tata's datacentre in the east end of London went titsup for two-hours on Thursday evening, following a power cut.
Backup power systems also failed, downing servers belonging to hosting providers throughout three floors of the Stratford facility at about 5.20pm.
Firms including C4L, ServerCity and Coreix were hit by the outage.
C4L's report to its customers said: "We found it very difficult to get a hold of our supplier as it appears they base their entire operations out of this data centre, phones where down and emails simply bounced back."
An engineer who visited the datacentre found that the batteries in its uninterruptible power supply were flat, and three generators had failed to start.
"We are understandably annoyed at this incident happening in the first place," C4L wrote, offering discounts to affected customers.
Grid power was eventually restored and servers came back online at about 7.30pm.
A spokeswoman said Tata was still looking into what caused the outage and the subsequent failure of backup power.
Stratford is the site of the massive Olympics development, so it's possible the power cut was caused by errant builders. We'll update this story if we find out more. ®