Maybe the website is down because of an ISP- or hosting-related issue. Try again in 10-30 minutes to check if it's back online. If it's still offline, report the issue to the hosting provider which will resolve it quickly.
Maybe the issue is in the server. Corrupt memory chips or disk sectors are known to be responsible for a large number of website-related issues and as a result of that, your website is unable to work properly.
First of all, check the memory chips and the disk sectors. If everything appears fine, the website's code might be the problem.
The best way to fix it, if you are not 100% sure, is to hire a professional to do it instead of you.
Note: If you still decide to try to fix it by yourself, check out this article.