If your website disappeared completely, it is most likely that you missed a payment for domain name registration or hosting. If you set up auto-pay, check that your credit card had not expired. Unless a long time has elapsed since your website went down, it should be an easy matter to get it restored. Another possibility would be if you fail to respond to requests to keep your registration contact details up to date.
If your site has just stopped displaying properly or has error messages, there could be other issues.
Problems can occur over time as software gets changed at any point in the chain (computers, devices, browsers, server-technology at your hosting company, etc). Sometimes changes need to be made to the code on your website to deal with new incompatibilities that arise. There’s more on this subject here: Why Does My Website Break?
If your hosting account is not protected with a strong password, hackers could find their way to your website and install malware. This is not common, but when it happens, Google is pretty quick to detect the malware and issue a warning notice when anyone opens your website. It’s usually easy to find the uploaded files and remove them (and you should then change your password).
Sometimes clearing your browser’s cache as well as the content management system’s cache can clear up minor problems. Try that first and if the problem persists, send us an email.