It sounds like someone factory reset it.

Many thanks - so how do I get it connected to my Sonos system again? I have been through the add product procedure and my Sonos does not find it.

You have a Play:5/gen1 on your profile. Is that still working okay?

Yes, that and another 2 x connect amps and a play 1 are all fine.

Good. In that case make sure they’re visible in the app, and try a factory reset on just the troubled Connect:Amp and see if it will then re-add.

An afterthought: are you sure the mute LED is flashing green, and not white? 

Yaaye! That’s fixed it thanks. Yes, it was flashing green but the reset procedure sorted it out.

Thanks again for your advice.

cheers