I logged off my computer

No i don't think so its a kind of virus or something. Just hit the ok button and you will get back to your desktop. Do not worry its not a virus.

Try ignoring the message by not putting a password & hitting the 'OK' button instead... Or try booting your system in safe mood... if it worked... go to your log-in/user account & check if your account is password protected... you can disable it from there... Or maybe it was asking for the administrator account password...