My guess would be some change in the environment, specifically around wifi interference , from an external source. Although there are other possible failures, too. 

I’d certainly reseat the cables between the CD player and the Port, first, then I’d unplug the Port and the Era speaker, and reboot the router. I’d wait a couple of minutes before plugging back in both Sonos devices to power. Then, after yet another couple of minutes, to allow them time to boot up, and connect to the refreshed network, I’d test. 

Thank you for this suggestion. I am.planning to try a reset tomorrow. This problem started after a 48 hour power cut - don't know if that had an impact, but I will try the reset!

Also, check that the default autoplay room for the Line-In on the Port is set to the Era 100 room name. 

If the reset works it may have been an IP address related issue.

If it happens again, usually after a power failure or firmware update, consider assigning your Sonos static/reserved IP addresses from your router’s DHCP Settings page.

I've tried rebooting my router and resetting my sonos system and the problem remains. Under system settings for my era300 speaker it says line-in disconnected, and grayed out. When I start line-in for my port, it shows cd playing through both my speaker and the port, but no sound comes from the speaker. I think I am missing an easy fix somehow. 

It may be time to call Sonos Support directly to discuss it.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

There’s only two things I can think of that would cause the Era 300’s line in to be greyed out. Or grayed out. One is that the device is set up as a surround speaker, in which case the line in is turned off by the software, and the other is that the line in isn’t receiving a signal. That could be one of many issues, from bad cabling, to a broken ADC adapter, to issues in the speaker’s electronics itself (extremely rare, but possible, it is electronics, and as such subject to potential failure). 

 The Era 300 is not going to show an active Line-in under settings when you don’t have anything directly connected to the Era 300.

Have you tried adjusting the Source Level within Port’s Line-in settings?

I've just tried adjusting the source level - no success. Is there something I can do to ensure the devices are synched? The app shows they are, but still no sound through the speaker

Is the Era 300 showing as being the Autoplay Room under Line-in settings?

Screenshots may help here to show what you’re seeing when you say the devices are synced.

Can you play another music source on the Era 300 to confirm that is actually connected to the network and working?

This is my screenshot

Yes, I can play streamed music through the speaker, just not cds

Have you checked within the Sonos app for any system updates?

Yes, I'm up to date

Thank you to everyone who offered suggestions. This morning I have found the solution. My two devices were evidently connecting to different Wi-Fi sources. I had connected them manually to the same WiFi, but it seems they automatically switch to whichever signal is stronger. Today I removed the second WiFi source as an option, so they are forced to connect to the same one. Hey presto, I now have sound through my era speaker. Thank you once again for all suggestions.