We have a theory about what happened - I was randomly logged out and went to log in with the key stored in my browser - but that has been there since last year and it's my master key which is no longer OK to use, so my account has been locked up.
I never knew about needing different log in keys because it's probably over a year since I last logged in, and I don't have copies of the other log ins - only the original master one
Hope it works out ok for you
Thanks - I've just posted response to all this here:
https://steemit.com/steemit/@sift666/have-i-been-hacked-or-blocked