I hear that familiar cry a couple of times a week. Sometimes followed by “what have you done?!”. Normally, nothing – it’s just your server, your connection or local server, or the internet itself.
There are a couple of great tools you can use to see what is happening with your site.
Down For Everyone Or Just Me? lets you know whether the problem is for everyone, or just you. If it is down for everyone then there’s a bigger problem.
Ding! It’s Up! will send you an email/SMS/Twitter when the site you specify is up, or for that matter, down.
You should also check the status of your server through your host’s tools if they have them. Also try to remember what the last thing you did was. If you were just writing a post and then it wasn’t working, well there’s a problem you haven’t caused (probably), but if you were changing plugin settings or a theme template file then perhaps undoing that last step may bring your site back to life.