On Feb 28th Amazon Web Services went down essentially breaking the internet, including Ether. When Amazon came back online each Ether username was “undefined”. Fixing this requires users to clear their browser cache by doing a hard refresh. For example, if you use Chrome for Mac hold COMMAND + SHIFT + R. For Chrome on Windows press CTR + SHIFT + R.

This should fix the issue right away. 

You can read more about Amazon going down here: https://techcrunch.com/2017/02/28/amazon-aws-s3-outage-is-breaking-things-for-a-lot-of-websites-and-apps/