Are you using a Upnp renderer? I had a similar thing where the DSM320 stopped talking to the network which kills the Upnp connection, which in turn prevents the SL connecting. If so... have a look at all connected Upnp connections on your network and make sure they're all showing correctly.

Have you also pressed the 'home' button on your remote to connect to NevoStudio? does wireless downloading work ok? Get this far and you'll at least know your SL commincating with the router ok.