Ridiculous MacBook Air Password Error

This morning I woke up and my Macbook is no more accepting my password, which I have been using for long years... I didn't change my password or else and it doesn't seem any other bugs can cause to this error. As if that wasn't enough, it does not allow to change my password via using iCloud. When I enter my iCloud info, it says this iCloud is not valid for changing password. I don't know what to do in this stupid situation. Do you know what's causing this?

[Re-Titled by Moderator]