what might cause a raspberry pi to just lose ssh capabilities like this?
You haven't actually provided evidence of that. It sounds like you have local control over the Pi (ie., a keyboard and display), so one check on this would be systemdctl status ssh.
If that is still running, most likely SSH is fine, the problem is other local systems cannot reach the Pi. A way to demonstrate that would be to ping the address/hostname you use with ssh. If that fails, it is probably because there is "no route to host". Unless you've intentionally blocked ICMP packets with a firewall, a running system will always respond to pings it receives, and the fact that they don't work means it is not just SSH that is unavailable.
While we're on that topic, you could try pinging another system from the pi. If you get a response, it is possible you will be then be able to ssh to the Pi from that system. If so, this supports my hypothesis below.
It was online, had an IP-address, and could ping websites, but my other home computers were not able to communicate with the pi and the pi couldn't communicate with them either [...] Have any of you seen anything like this?
Yes, and eventually I traced it down to what I think was an issue with DHCP leases and ARP. Basically, the router will still relay packets to the internet, but the Pi cannnot properly operate across the LAN.
My temporary solution to that is to try:
sudo systemctl stop dhcpcd
sudo rm /var/lib/dhcpcd5/*.lease
sudo systemctl start dhcpcd
If that works, there are various things you can try in dhcpcd.conf.
If you aren't using dhcpcd, there will be some parallel methodology: Shut down your network connection, find where leases are stored, delete them, restart networking.
/etc/dhcpcd.conf? As a 2nd guess, are your/etc/hosts&/etc/hostnamefiles configured properly?systemdctlcommand below since the problem is resolved, but I’ll definitely keep it in mind if I see this issue again.n I was unable to reach the pi,