Do you know the name of the mail servers you wish to connect to? eg. pop.gmail.com?
If you then try the following
Open a command prompt (Start > Run and type cmd or command for 98/95) and type the following
ping pop.gmail.com (or whatever it is)
if you get a destination host not found or any other message instead of a "Reply from..." then it looks like the DNS provided by your ISP is down or has changed..
Let me know how you get on..