getting a play 3 back on line. dont know why it left in the first place

That isn't enough information to be of any real use to the folks here that would try to help you.

Generic suggestion, since we are lacking specific information: Power down all Sonos, reboot router and controller then power the Sonos back up.

More info would help us if that doesn't work. 

Don’t Factory Reset for an issue such as this. The Factory Reset will cause extra work for you and will not resolve the fundamental issue. I suggest that you submit a diagnostic within 10 minutes of an event, then work with SONOS phone support. Issues such as this are often due to local network issues. I realize that this speculation often rumples feathers because users feel their network is “perfect” because other units seem to work OK. SONOS requires more robust network support than simple web access.

I’ve seen a few similar reports. It’s important that you work with support because this will result in the issue being properly logged and reported to the developers. This will allow support to differentiate between local network and SONOS software issues.

Capture a copy of the error message image and post it here, don’t post copy of the diagnostic number here.