There are many reasons a web site might go down, but the most common reason is operator error. A designer might put in the wrong code, or put it in the wrong spot, and not realize it. Then they publish the changes and break the web site.
This is the easiest to fix since all the designer needs to do is fix what they broke.
Web sites can also go down if they are hacked. This is a serious issue because a hacker can do all sorts of damage.
The first thing to do if your web site is hacked is to change all passwords immediately. Once the site is secure, then go in and restore a backup.
If your web site continually gets hacked, even after you change passwords, change hosting providers immediately. Chances are, the hacker works where your web site is stored. A person does not need your login and password if he/she has physical access to the server.