'404' errors are usually generated by the server that your computer is trying to connect to, rather than by any software on your computer. That strongly suggest that the problem 'is at their end' (i.e. with your email provider's website) rather than 'at your end'. So, apart from waiting for your email provider to fix the problem, there's probably nothing that you can do.
However it
might be worth trying to access the site with a different browser (such as Firefox if you're currently using Internet Explorer):
http://www.mozilla.org/en-US/firefox/all/
You could also try getting in (via 'POP3' or 'IMAP') by configuring an email client (such as Outlook Express or Windows Live Mail) but we'd need further information from you in order to tell you how to do that.
So, if you post again, please tell us:
(a) your operating system (e.g. Windows XP or Windows 8) ;
(b) your web browser (e.g. Internet Explorer, Firefox or Chrome) ; and
(c) the name of your email provider (i.e. the bit after the @ symbol in your email address)