Whenever someone tries to enter in the company's admin page, the browser returns an error message, "This site can’t be reached". This error message also appears when trying to access the Front End website as well.
You really haven't given enough information to give a real answer. You're going to have to check Magento's error logs, and if you can get to the server error logs, you're going to have to check those as well.
- Have you tried accessing your site over FTP or SSH?
- It could be a resource issue; your server might not have enough RAM, CPU or disk space to load your website.
- It could be a DNS issue, has anything related to your domain changed?
Start with trying to get access to your logs and seeing what errors are there.