We had a mishap with our Mac Mini M1 server over the weekend of 11/20/2021 - 11/21/2021 and experienced the first extended downtime with this server. Fortunately, it wasn't a serious issue, but since it was over the weekend and it essentially disrupted any email contact, it lasted much longer than it should have. I realized our server was not responding about 1:00 pm our time, and submitted a trouble ticket to our colocation facility about 15 minutes later. Since the contact verification bounced (because my server was down), things were delayed. I've since added an additional contact outside of our server-controlled emails to hopefully avoid that problem again.
Cutting to the chase, the culprit was my Time Machine backup disk failed, causing the system to crash. I still have a problem with a non-system disk failure causing a system error, but that's what happened. The Time Machine disk is a rental disk from the facility, so it's essentially swapping a $150-ish disk with another $150-ish disk and we're right back where we were, minus the previous set of backups. I also keep a pair of disks offsite that clone the main disk remotely each night, so it would take a couple more points of failure to really make a mess of things.
Anyway, crisis resolved, and something to keep in mind if I see similar symptoms again.