What client are you using, a few people seem to be having problems using Microsoft Outlook at the moment.
Does webmail work ok
BT email randomly going AWOL has been an unexplained problem for years. It happened this morning, when I was temporarily unable to connect either with Thunderbird ("password issue") or to webmail ("account unavailable"). As usually happens, it righted itself spontaneously, and some have reported that an email client failing to connect can be resolved straight away simply by logging into and out of webmail.
Try again now if you haven't already.