There could be a range of factors or problems to work through.
Is your internet working properly? (Check to see if other sites are loading correctly. If not, it is likely your internet that is the problem, not your site.)
Have you paid your hosting provider? (If not, your account may have been suspended.)
Is your hosting provider experiencing technical difficulties? (This happens from time to time. Outages are rarely long and may just require patience. Even the big boys experience this time to time - and to some degree, it is just part of having a website.)
Have you been hacked? (Let's hope not, but even the most secure institutions get hacked from time to time. If your hosting offers a backup service, then the easiest solution is often to restore a backup from before the hack.)
Has someone logged into your website and inadvertently removed or unpublished it? (If this is is the case it will usually show an 'under construction' notice. If this has happened, log in to your admin area and republish the site.)
Contact us if we manage your hosting. If your site is down and your hosting is with someone else, you will need to contact your hosting provider.
If there is an issue with the general function of your website, then please submit a ticket and we can take a look. Please note, fixing a site that is no longer under warranty, or is having a problem due to changes you or your team have made incorrectly will incur a fee.