OP answer: BITDEFENDER developed a sudden hostility to multiple ExpressionEngine sites, many of which I access on a daily basis. When I turn off Bitdefender I am able to access all EE sites again. If you run into this issue save yourself some time and check your anti-virus software first!
It could be a stale cookie. Clear your browser history and cookies then try logging in again.
Otherwise open up /system/user/config/config.php and add:
$config['cp_session_type'] = 's';
This forces control panel logins to use session ID's instead of cookies.