There used to be a website called something like Whistle Blower (although I
couldn't find it with a quick search). It was free and all it needed was a
static IP address which it would ping regularly. If/ when the ping failed it
sent you an email.

I needed it to keep tabs on a baulky Netware Web Server that crashed almost
nightly but haven't used it for 12 years now. It may be gone but surely
someone must be providing a service like that.