It sounds like perhaps either your domain name annual fee was not paid, or perhaps your hosting, maybe even an SSL certificate has lasped. It’s important not to lose your domain name as it gets put back into an auction and some people auto bid on expired domains to then charge you more if you want it back. Try first to phone your provider – but if they are no longer an option then someone ask independent to look at your situation. If it is SEO only, you would still see your site if you type the URL into the browser.