There’s several potential reasons. It may be that your new WiFi doesn’t match the Sonos Network Requirements . Or it may be as simple as following the new router - installation instructions FAQ. Or even just using the same SSID and password on your new router as was used on your old router. 

Sometimes it helps if you temporarily connect via Ethernet cable… 

I had the same issue, and it worked when I connected the Ethernet cable directly.

This software is unreasonably difficult to set up. I changed wi-fi, and had to read several articles to figure out how to reconnect. Then found out the ‘direct from my phone to the speaker’ was discontinued. WTF? Who thought that up?  And no, I don't have a Bluetooth unit, so what, upgrade? Sorry no way. I was an early supporter, but not any more. Units are great sound, Software system is a total joke. And features are dwindling.

This makes no sense to me. You’ve told your Sonos to connect to WiFi X. Then you take away WiFi X and install WiFi Y. How can Sonos connect to WiFi Y without you connecting it to an Ethernet cable, so up the Sonos can be on your LAN and be told the new WiFi credentials? It would still be looking for, and trying to connect to the WiFi X that you told it about?