If you run a business, your website going down is the kind of thing you'd rather hear about from an email than from a customer. Companies charge monthly for that email. This does the same job from a Google Sheet: it checks your sites every few minutes, emails you when one stops working (including when its security certificate expires), and emails again when it's back. No account, no card, no subscription. You build it once and it runs.
One script watches every site you list. Five sites or fifty, it's the same setup, and adding another is one more line.
Here's what it looks like running, with sample data.
Every few minutes the script loads each site the way a browser would. If a site answers, that's a pass. If it errors, times out, or its security certificate has expired, that's a failure.
One failed check doesn't email anyone; sites hiccup. A second failure queues a confirmation two minutes later, and if that fails too, you get the email. Real outages reach you within a few minutes and momentary blips never do.
If every site fails at the same moment, the problem is almost certainly the checker's own connection, not your sites. You get one email saying so instead of one per site.
From here on it's yours. List your sites and who should get the email. The script below rewrites itself as you type; then it's one paste into Google and one click.
Code.gs below and save.status.html where marked and host that file anywhere.