If you are experiencing specific problems with one account, yet others work fine on
the same computer! It is very likely a server side problem.
If you have access to another computer see if your account can be signed in on that.
If you can't this can not be repaired by yourself and you will need to provide the Messenger support team - with very specific information of the problem and how you have been able to sign in, on the same computer, to other accounts by e-mail, they may be able to help. A support form can be found at
http://support.msn.com/eform.aspx?productKey=mess...byemail&ct=eformts