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.  You're better off creating a support ticket or calling support, unless you're a server administrator and the server is accessible.

2.) Network outage.  This is the second most common cause.  Unfortunately when there's a network related outage the problem may be with your web host, internet service provider, or a jump/router that sits between the two.  These are definitely the most difficult to resolve.  You can easily determine if the cause is network related by running a traceroute to your website.  A traceroute will allow you to follow your connection from your computer all the way to your website.  If it fails prior to getting to your web hosting company's network then you know it's definitely network related.  To resolve a network outage you should contact your web hosting company and provide them with the traceroute that you created.  This will allow them to determine the exact point of failure so they can start the repair process.  Another thing that you should include with your traceroute is your IP address.

3.) Domain name has expired...  We see this all the time!  A lot of our clients use various domain registrars for purchasing or renewing domains.  They often forget when they're domain renewal is due and the registrar suspends the domain.  The maximum term that you can register a domain name for is 10 years and domain registration is rather inexpensive.  I strongly suggest that you register/renew any business critical or important domains for a minimum of 5 years.

4.) Your hosting company has gone out of business.  I know this sounds horrible, however ITX Design has been in the web hosting market for over 10 years.  In that time we've seen countless hosts close their doors with little to no warning.  You should always have a backup solution even if it's a <a title="cheap disaster recovery for your website" href="https://itxdesign.com/cheap-disaster-recovery-for-your-website/">cheap disaster recovery plan</a>.