Can't reach firewall - what can I do?

[Asperamanca]

I have used a Opnsense-based firewall (ready-to-use box from Deciso) for 3 years now, and regularly updated the OpnSense version.
Today, I simply cannot reach the firewall anymore (which I also use as DHCP server), my PCs don't get valid IP addresses. Even if I manually force them into the correct subnet, I cannot reach the firewall.
I checked the troubleshooting section, but all articles there seem to assume you can reach the web UI.

What can I do?

Edit: It's a DEC 600 and I already tried a hard reboot.

[dseven]

Sounds like a hardware failure. Can you connect the USB/serial console and see if that reveals anything? https://docs.opnsense.org/hardware/serial_connectivity.html#serial

[Asperamanca]

I see a COM port appearing in the device manager when I connect Windows PC to the DEC via USB cable, but when I try zo open a session using PuTTy on that com port ( using elevated rights) nothing happens.

[Asperamanca]

Argh, network cable slipped out. Fatal User Error....