Unknown Host

Unknown Host

We are unable to send or receive mail through Zoho using our Office365 server this morning. The error message is "
The Server could not be connected. Check whether the Server is available to connect in the given name and port number.

The account is disabled now, and when I try and enable it, it says that outlook.office365.com is an unknown host. 

It has been working fine for months, until this morning. 

Ideas on how to fix it?