Google had a global email issue, over the last 48 hours, affecting Google Apps for work customers, mainly in the US and UK. It can affect isolated users within Apps domains. The symptom is a ‘service not available’ message when the user tries to log in, as though email were an optional app not enabled for that user.
The fix is very simple. I’ll prefix this, though, with the disclaimer that you’re responsible for your own actions and data and should take advice, by using the support system in the domain management console, if you are at all uncertain or want to check this with Google first.
In the management console, choose Apps and then Google Apps. Gmail should be marked as ‘on for everyone’. From the three dot menu to the right, choose ‘Off’, then click to confirm in the dialogue that pops up. Wait ten seconds, then click again on the three dot menu and choose On For Everyone, assuming that was your setting. If you only allow it for selected users, enable them as you did previously.