Without more detail, it sounds like you may have set up two different ‘systems’, one with the ‘first’ group of speaker, the second with the pair of PLAY:5s. If you unplug the PLAY:5s from power, then open your controller, do you see the original system? if so, you’ll need to factory reset the PLAY:5s (likely again, as you would have to set them up as two separate systems), and then use the ‘Add a device’ process in your existing system and controller to add them back in.
However, if this isn’t the case, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the Support staff, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.