The sloppy housekeeping represents an embarrassing lapse for Microsoft Corp. as the software maker tries to bring in more revenue from the storage service, which is called Azure.
The expired certificate is needed to properly run online services such as Azure which use an "https" protocol to block unauthorized users from accessing information.
Microsoft's failure to renew the security certificate apparently caused the Azure service to go down shortly before 4 p.m. EST (2100 GMT) Friday. The breakdown prevented Azure customers from accessing files kept in Microsoft's data centres.
The service still hadn't been fully restored more than four hours later, according to a post on Microsoft's website.
"We apologize for any inconvenience this causes our customers," Microsoft said.
Azure's failure illuminates the pitfalls of storing important information in remote data centres. Online storage, often called "cloud computing," is growing in appeal because it allows workers to pull up data, wherever they are, to an Internet-connected device.
Cloud computing's convenience can turn into a major aggravation when a problem crops up like the one that tripped up Microsoft Friday.