Invalid username? Can′t log in

Hello everybody,
I have a quit simple problem: I can′t log in, WP says I have a invalid username. It′s strange, because I didn′t change anything, no upgrade, no new plugins, no moving, nothing, and yesterday it was all fine. It was safed in my browser, so I only have to type the first letter and it completes the login form. How this can be, how my username can change alone? What can I do?
I downloaded login.php to see if there I can find any option, but not. Is there a php file, where I can see this detail? Without any problem WP send me a new password for my username, but on try to log in it says everytime that the username is invalid. I read a lot of answers to this problem, but didn′t find a solution. Please help me.