photo credit: Bull3t
The www prefix is essentially a subdomain of your website and although many people will type it in, some may just try to enter the domain itself.
Many websites, including high profile ones, can’t be accessed without the www subdomain. For example, barclaycardunwind.co.uk is inaccessible while www.barclaycardunwind.co.uk is.
This is essentially a DNS or server configuration problem, so a single solution is not available for everyone but it should be straightforward. Contact your web host if you are not sure or modify your server configuration if you know what you need to do.
Once you can access your site without the www, make sure that the traffic is forwarded to one or the other to prevent potential duplicate content issues with google.
Make sure you do it, or risk frustrating and/or losing a lot of your visitors!