Signing Into YouTube In 2013

You cannot sign in using your YouTube username (the bit after com/user/ in the URL) as before.  Now you must use the email address your YouTube channel uses via its Google account.

a)  If you are not sure what Google account using what email address your YouTube channel is associated with (and also if you have a non-linked legacy pre-May 2009 YouTube as that will prompt a linkage interface for you) then use the YouTube username and password as before and you will be told what Google account's email address you should be using:

b)  If you you do not remember your password, or are told that the username and/or password is/are wrong when trying a), then you need to use the "Need help? option instead as shown in the diagram.  This will give you your reset options depending on what you have set up for yourself.