Hmm, well firstly I'd reboot both the router and phone
Second, check to make sure its really WEP and WPA2 or something.
Third, check the router firmware.
Fourth, check to make sure you aren't filtering on MAC address or that you haven't somehow run out of addressable IPs.
Fifth, check to see if the router is broadcasting in b, g, n, mixed, or what have you. Try setting it to just one and not mixed and see if that helps at all.
Sixth, I ran into a case once where a specific model router was not compatible with my Zune HD (which makes no sense to me since I thought there were standards...), but suffice to say I ended up having to wait a month for a patch. In the meantime, I had to use a different router :/