Your website is probably down because you're NOT using ITX Design for hosting...

On a serious note there are only a few things that can cause your website to go down and I will list them in order:
1.)  The server that your website is on is either down or one of the critical services is failing to reply.  This is one of the most common causes and if this occurs the best thing you can do is contact your web hosting company for support.  Your better off creating a support ticket or calling support, unless you're a server administrator and the website is accessible.