If your site 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 site 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.
If your web account is not protected with a strong password, hackers could find their way to your site and install executable files. 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 site. It’s usually easy to find the uploaded files and remove them (and you should then change your password).
Other 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 site to deal with new incompatibilities that arise. There’s more on this subject here: Why Does My Website Break?
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.