There's quite a big list of dependencies when dealing with running your own web site. Obviously you're busy adding content and doing all kinds of Search Engine Optimization - but there are a couple factors that can be a threat to your web site and it staying up.
Here's a short overview of things that can (and most likely will, at some point) go wrong!
If you're at the point where your site is important to you - consider getting some Server Monitoring Services, offered by most high-end web hosting companies... Some private hosts like yxyservers even offer SMS monitoring and 24/7 web cam view of the server in HD.