Since 9:00 AM Today users started reporting that they were unable to gain access to their Google Apps for Business accounts or Gmail for Work. Google Hangouts is also affected.
At this time it is not clear when the service will be fully functional however Google has a link to their official status page.
The outage was first reported by DownDetector who reported that it was a service wide outage.
As of 3:40PM Google still reported that Gmail service had been restored for some users, and that they expected a resolution for all users in the near future however in their own estimate that may change and that delivery of messages sent to affected users during the issue may be delayed.