When this next happens try pinging the router, you will need to know its IP address and to be root.

eg:This way you will know whether it's your phone (you cannot ping it) or your router (you can ping it)

I had a similar issue where the phone would show it was still connected to the Wifi but all communication had failed, switching off and then back on Wifi in the phone (using Wifi switcher) would fix it, but I don't think it has happened since I re-flashed, so maybe a reflash would fix it.