Cannot open Youtube

Hello,

it was still okay, but all of sudden I couldnt open Youtube at all in any browser (IE und Chrome)

It is written

Access to the webpage was denied
You are not authorized to access the webpage at http://youtube.com/. You may need to sign in.
HTTP Error 403 (Forbidden): The server refused to fulfill the request.

Dont have any clue at all. What does it mean by signing in? Pleasee help, thanks.