This happens when something goes wrong while the device gets its IP using DHCP and (repeatedly?) fails to do so.
Quick and dirty - restart your router, 'forget' the network, reconnect.
If that doesn't help, or for proper fix, check your router DHCP settings, are there enough IPs for everything in your LAN? Is MAC filtering on, and if it is, is the phone MAC allowed? Etc.
If everything else fails, assign a static IP to it, one that belongs to your LAN net mask, but outside of DHCP range.
Just enable it :-]