We'd love to help you solve this problem.
First we'll need to get some more information.
Can we check out the blog? If you give us its URL, we'll know exactly what you mean by "doesn't exist at all". We might even be able to find some hidden information that could help us figure out what's gone wrong.
Is there any error at all? If we can't check out the blog, we'll need to know if any information was displayed at all. If you got an error code, like 404, that's one thing; if you get a blank page, that's another. If you get a page, even a blank one, use "view source" in your browser to see if Serendipity even tried to tell us anything.
Can you still get to the admin page? If your blog is normally
http://somedomain.com/somepath/index.php, your admin page will be at
http://somedomain.com/somepath/serendipity_admin.php. We've done our best to make the admin page as robust as possible; even when there are minor problems that affect the blog, the admin page will still display.
Do you know what version of Serendipity you're using? If not, we can usually find out from the admin page.
With that kind of information, we'll be able to figure out the next step to take. Unfortunately, many cases of blog disappearance occur when the web host provider deletes the account -- for whatever reason. If that's the case, your only recourse is to contact them.
Sorry to hear about your son. I hope he's okay.