I was a ways through the Admin Trailhead.  After submitting a challenge and error was encountered (sorry I don't remember the error message).  I logged out and then logged back in.  After logging in an error message was displayed stating that the trailhead servers had been overloaded and that I should click the refresh button.  I tried this several time, but to no positive avail.  Finally, I logged out and logged in again.  I was able to log back in, but all of my previous Trailhead progress was gone as if this was a totally new account.