I would like to post a tribute about this service I use to keep under control this site.
"DownNotifier sends a free alert when your website is down". You fill a form and you're done.
Even the basic plan, which is free, works pretty well. I've been using it for 1 year and I'm very happy with them.
If you own a website, pay them a visit:
https://www.downnotifier.com/