Why does domain not work when 'www' used

[DirectAdminFan]

I've been looking for any prior threads on this but didn't see any. I recently created nameservers, added the domains in Direct Admin, and everything was working fine. All set up.

Then suddenly, my website was no longer accessible when using 'www'
It is now only accessible when typing in the domain name without 'www'.

So for example, if my site was Direct Admin:
http://www.directadmin.com would not work

but http://directadmin.com would work.

Does anyone know why this would happen? Because everything was fine before and then it just stopped working with the 'www'.

Thanks.

[hehachris]

sounds like a DNS issue to me.
do you have a www entry for this domain?