Does the Dune get its IP dynamically - ie is it given out by your router automatically or do you have a static IP address set?
Have you checked all cables between router & Dune? You should get the lights on the network ports lit if they can see & talk to each other.
Are you hard wired with Network cables, via Homeplugs or using WiFi?
Do you have a Netgear router and/or network switch? Some Netgear equipment is reported by some to not play nice with Dune media players, but not sure why as it has always worked well for me (router and switch).
What has changed on your network recently that could have caused the problem. If any equipment has been moved it may be worth removing the network cables and reseating them to make sure they are not loose.
Have you tried powering the router off and back on? Sometimes a router can run out of dynamic IP addresses and a reboot resets the DHCP server.
Good luck with the above and let us know how you get on.
Mark.