Looks like it's back up. Their web site was returning a 502 Bad Gateway for about an hour and the General availability on their status site https://www.fastmailstatus.com/ was "The service is unavailable".

Sent emails to fastmail, went through right away. UI appears fine to me. Only 2 reports on downdetector [1]

[1] - https://downdetector.com/status/fastmail