Does Sonos work with wi-fi? I have a similar product (Bose Soundtouch) and that works with Bluetooth, not wi-fi. It does use wi-fi for other stuff (e.g Internet radio) but not for streaming music between a player and the speaker ...
Possible errors would be:
1) the Sonos has forgotten your wi-fi password
2) the Sonos is positioned at the edge of wi-fi range
2) the Sonos cannot get onto the network, possibly because there is not a free "slot" for it. In such circumstances you can usually log in to the router and reserve slots for pre-defined MAC addresses. Instructions for doing this would vary according to the exact router you had (there is more than one home hub).