Big thank you to ejb5oh for the rollback advice.
Awesome.
I was logged out after upgrading to the latest version of ithemes security, and then re-applying the hide background as advised.
This unabled me even to find the login screen, as it came back as a 404 error.
I removed the plugin via ftp, and re-installed version 3.6.6 and hey presto, I was able to log back in again.
Cheers.
