If you have physical access to the power button… can you connect a monitor+keyboard? Or if it’s a server use the remote-management port? I’d expect the console to always work.
If you can get in… it may be a memory issue specifically for networking: I’d look for dmesg output that says something like “TCP: out of memory” (like here). So perhaps sshd is still running fine… but TCP/IP connections simply can’t be created anymore.
(Edit: but your guess is reasonable too. You may have to just reboot it and go through logs when it comes back. /var/log/syslog may have one or more smoking guns)