If you entered an e-mail address when you registered for Skype, you can request a new password by filling out a form after which a new password will be e-mailed to you. Go to https://secure.skype.com/account/password-reset-request and fill in the form.

There could be any number of reasons, including that someone hijacked your account. Have you gone to the password recovery link to make sure that the password has not bee changed somehow?

If you can't log onto your Skype account I would recommend that you go to the page linked below and enter the email address for you Skype account, you will then get an email with a password recovery/reset link, where you can set your password.
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