This is a rare bug where some users are no longer able to log in using their username and password on specific devices + browsers.
The bad news is we don't currently have a timeline for the fix for this. The good news is that there's a workaround!
1. If you have a different browser on your device, you should be able to log in with your email / username on it. So if the bug appears on Safari, try using Chrome. If you're fine with using this secondary browser for Khan Academy going forward, you're all set!
2. If you'd prefer to use the browser you're locked out of, you must connect your account to Google or Facebook.
In order to do this, go to your Settings page and scroll down to Linked Accounts. Choose to link your Google or Facebook account with your Khan Academy account. Then you'll be able to log in using either of those accounts on the browser that you were initially locked out of.
April 28th 2016
Internal ticket: 104390780845190