Uptime Robot is a monitoring service which basically pings your web site periodically and emails you if it's down and when it comes back up. The free account includes 50 monitors and 5-minute interval pings. I vaguely recall when I signed up a few years ago it was one monitor pinged daily so 50/5 is overkill. And if I look at my entry now it is set at 24hr ping.

Mostly I was using it to see if my IP address had changed since internally I can tell if the server is up or not but IP address changes can go for a week or two before I notice. That's not a problem any more since my TP-Link router has dynamic DNS so will update my DNS entry pretty quickly.

With the admin console you can see history of outages and pause individual monitors. Many other things require paid subscription: SSL certificate monitoring so it warns you when it expires, alerts for slow response times, maintenance schedule so it won't falsely alert, sending specific http request with login details and request body, connect with a third-party service like Slack.

For a free and basic "tell me if my server is not reachable" Uptime Robot works just fine.