Hi,
A common mistake is that you must use the entire address as your username in your mail client:
i.e.
you@domain.com as the username
Also, you may want to try to access your mail using Webmail and see if that is working.
Check
http://www.network-tools.com/nslook/ and see if your domain is pointing to the correct server and mail server.
Finally, if you can post the error you get in your mail program (if any) someone may be able to help further.
Regards
Tom