Turn off the Roam.

Unplug all your other Sonos. Reboot router and controller. Power up all but the Roam.

Start the controller and see if you have your system back.

If so you likely ended up setting the Roam up as a new household, instead of adding it to your existing one.

As @Stanley_4 mentions… if you did ‘mistakenly’ setup your Sonos Roam as a new Sonos Household, you might have also reset the S2 Sonos App (or reinstalled it) if that did happen at some point, then you may need to reset, or reinstall your Sonos App.

When opening a reset Sonos App, on a device connected to your Home WiFi, you need to accept the T&C’s and go through the opening screens until you see the option to connect to the ‘existing’ Sonos system on the local network - select that option. Do not create a new system and at this stage do not factory reset your Home Sonos devices.

Get your system back up and running first of all and then go onto add your Sonos Roam to your home setup (you will likely need to factory reset that one device only to achieve that).

When that’s all sorted and everything is working at Home, next go onto add your mobile home (other) WiFi credentials (SSID/Password) to your Sonos Roam via the App ‘Settings/System/Network/Manage Networks/Update Networks’ and follow the onscreen instructions … This Sonos Support Link below also provides the details on how to add a network to your System & Roam device…

https://support.sonos.com/en-us/article/connect-sonos-to-a-new-router-or-wi-fi-network

Hope that helps you to get things back up and working for you.

Edit: When all has been fully resolved, your Sonos Roam will then work either on your Home WiFi, or on the Mobile Home WiFi network too using the same Sonos App.