Sub and amp cut out, no sound

While you wait on someone to look at that diagnostic, you didn’t by any chance turn off the wifi/radio on the Amp, did you? That would cause it to be unable to reach the Sub for playback purposes.

No, I haven’t made any changes for well over a year to the system - it happened all of a sudden.

Are you able to ‘group’ the Amp with another Sonos Room and play audio to the group?
Have you tried ’bonding’ the Sub to one of your other Sonos Rooms? - because if you try these two things, it may just point to the problem as perhaps being your Sub, rather than the (reset) Amp …

If the tests do point to the Sub, then I would maybe try resetting the Sub too and/or wiring that temporarily to the LAN to just see what happens.

“All of a sudden” certainly suggests either some sort of wifi interference between the speaker and the Sub,  or a failure in the electronics of one of the two speakers. I suppose when someone at Sonos looks at your diagnostic, we’ll know which one it is. Failures of the hardware are rare enough that I’d be expecting some sort of wifi interference, potentially outside of your home network. 

This blew my mind!

First of all, yes: I have a couple of speakers wire connected directly to the amp, as well as two Sonos One’s through Wi-Fi, and all that works completely as it should!
Now what blew my mind, is when I simply “moved” the sub from the room with the amp, to another room with just one Sonos One, it started playing instantly.

I tried moving it back to the room with amp, and it turned off again.

I suspected a wifi interference, but no changes in my network has been made - yet I have done a software update on my router and reset on both the amp and the sub - to no avail, unfortunately.

The sub works, see my most recent answer to Ken.

Does the Amp still work, if you remove its Ethernet cable? Also, just for clarification, when you moved the Sub to the other room, was it bonded to the Sonos One when it was playing? Then when you ‘bonded’ it back to the Amp it then stopped playing - is that what happened, or did it just simply stop playing when you changed its actual physical location, rather than after switching its ‘parent’ (bonded) device?

Your network is equally impacted by outside influences as inside influences. Whether or not you made any changes has little to due with their being an issue. Electronic devices fail, and cause interference, I once had a microwave that caused my kitchen speakers to fail while it was on. I’ve also had a new neighbor move in who was using a frequency on their router that ended up causing interference on my system. In neither case did I make a change to my network before experiencing the issue. 

Yes microwaves can be problematic for any wireless network device and another problem is baby monitors (although that’s not likely going to be an issue these days at my age..).

For many though a simple reboot/auto-upgrade of the router can mess things up, particularly  if ‘auto wifi channel selection on start up’ is left in place on the local, or a neighbours router and that is often the case.

My son-in-law’s BT smart-hub router had an update recently and that messed things up on his home network, that we had to sort by manually changing back a router setting that had isolated his 5Ghz network. Anyhow I am quite sure things do change in most networks, from time to time, without any of us lifting a finger.

Especially as many routers frequently update their firmware without any interaction from us ;)