Try to login through the console.
Press CTRL+ALT+Backspace(the X envoirment will shut down now) and type your account name and password. If you logged in successfuly it will bring you something like “Login succesfuly!” message. Then you just have to type “startx”, “startkde” or “kdeinit”- all these make the same action - starting the graphic envoirment and in your case it’s KDE 4. If you’re not logged in, then you’ll be able to see a system message with the error and find out if the problem is in your graphic envoirment or it’s a problem with the account, privileges or something else.