If it's a BT Yahoo account you should be able to access your mail by either using web-based access or (usually better) using an email client, such as Outlook Express or Windows Live Mail. If you can't get in one way, try the other.
I haven't been able to get onto yahoo for two days, just kept saying 'unable to connect to server' or 'server not found' from firefox and IE. It came back yesterday, seems o.k. now, touch wood.