The status LED lights are explained here in this Support link: https://support.sonos.com/s/article/226

Maybe first see if the speaker works when it’s wired to your router. If it does then check for any Sonos updates and re-enter your WiFi credentials to the speaker via ‘Settings/System/Network/Manage Networks/Update Networks’ in the App

Meanwhile I plugged in an Ethernet cable, rebooted the router and the sonos as well.

Unfortunately nothing changed. I think that is not about connection because it is not working with Ethernet cable either.. The flashing white led maybe indicates that the device keep booting up but actually it is never able to finish it? 

Any attempt to hardware reset the device by pressing the connect button and plugging the speaker does nothing. 

I am completely lost now. No idea what could I try to bring the sonos back to life

To factory reset you must press and hold the Join/Connect button on the back while plugging in the speaker, and keep pressing until the LED flashes amber/white. After a short while the LED should flash green when it’s ready to be re-added.

As you’ve already tried the factory reset procedure and have not had any joy with that, then your next best option is to perhaps contact/chat to Sonos Support Staff.

Yes, as @ratty mentions, make sure you are doing the ‘reset’ procedure correctly. See the ‘factory reset procedure’ hyperlink in my last post.

After the correct reset procedure the situation is the same :(

I am holding the connect button for more then 5 minutes but probably it's no use to insist more..

Thank you very much for all the help and suggestions!  

The product it's still in warranty, so maybe the best idea would be to send it back to be fixed, whatever would that mean. 

If it’s broke and still under warranty, then contact the Sonos Support Staff via the link provided earlier, to get the device replaced.