Why is my iMac with Big Sur not seeing wifi networks?

Today, my iMac with Big Sur 11.1 decided it would not connect to our wifi networks. So I turned wifi off, and then on again, to see if that resolved the issue. Nope. Now I can't even turn wifi on. If I go to System Prefs and click the wifi switch, nothing happens. It stays switched off. None of my networks are listed, and the wifi icon on the menu bar is greyed-out and has a slash through it.

I am now running an ethernet cable direct to the modem to maintain a connection. All of my other devices (iPhones, iPads, MacBook, TV, Playstation etc) can connect fine, there is no disruption, only the iMac.

Here's what I have tried so far:

• Rebooted computer
• Rebooted modem
• Rebooted routers (I have multiple wifi networks in parallel but none are visible)
• Deleted plist files from system preferences folder and restarted
• Created a new wifi configuration in System Prefs
• Reinstalled a clean version of BigSur

None of these have resolved the issue or changed the symptoms. Curiously, the wifi switch on my menu bar is ON, but with no networks displayed. In System Prefs, wifi is OFF, and can't be switched on.

Help??