Apart from the device you factory reset (not recommended), your system is still there as all settings are saved on each device. As long as you haven't reset all of your products, it's just the app that can't connect.
Assuming your network settings haven't changed, can reboot your router, followed by all of your Sonos devices. If your system connects to the App, try reconnecting your reset device
@Maciej001,
I would power off the device you factory reset earlier and set it aside. Reset the Sonos App and connect back to your existing Sonos System and when that’s showing in your Sonos App. With all your speakers showing in the App, then power on/factory reset the problem speaker and add it back to your system using the ‘Add Speaker or Component’ option in the App Settings (gear icon top right).