My PRTG Instance Failed Last Night and Needed to Be rebooted

My PRTG instance needed to be rebooted this morning before it would start working properly. I had sensors that were flipping between up/down (I have a primary and failover configuration) and Unknown. It appeared to occur between 12:40ish this morning and 6:14am when we restarted the server. I have a notification configured to send an email to an email to phone service and they in turn call a group of phone numbers that map to a group of support personnel on my side. The notification is configured to run if there's a down state for 300 seconds. There was a server that was in some sort of down / unstable state and was definitly now working correctly. The HTTPS sensors for the websites on that server should have fired off a notification for the email to phone feature that I just described. It didn't. When looking through the logs, PRTG was flipping between Unknown and up/down status. Should PRTG send an alert when a sensor is Unknown or Unknown / down? The key issue is that I had a server down for about 5 hours last night and PRTG didn't fire off the email to phone notification. Once the server restart was completed I similated a failure and waited for 300 seconds. It worked perfectly. I will say that the PRTG server was waiting on a restart for a Microsoft Windows update it had received the week earlier. I"m not sure it it had anything to do with the issue. Please advise.

thanks,

Todd