a domain name links your site to the adress you enter in the browser url-bar (like www.example.com) and is usually only leased for a certain amount of time.

first of all you should inquire if this is a legit message by checking if your domain has indeed expired or will do so shortly. use a site like http://www.internic.net/whois.html (or one that applies to your countries TLD) to search for your the whois info of your domain & check the expiration date and registrar. if expiration is due and the site you are redirected to belongs to the registrar, then you should indeed make arrangements with your domain name's registrar to renew the adress.

To be sure that it isn't a problem with the cache (it may have been fixed already) you can try to clear the cache.

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"