If a reset doesn't get it back to normal, it's probably defective. My
D-Link died after 2 years of use (the typical life expectancy of
D-Link's entry-level NAT routers). Any computer connected to it, even
if the WAN-side wasn't connected to the cable modem, would result is a
high-rate flashing of the port's LED. The Link status in the OS kept
changing from connected and not connected. Resetting it many times and
in different ways didn't help. Replacing it fixed the problem. I've
seen 5 D-Links die in the last 4 years and all were around a few months
plus or minus of 2 years. I think they have an overheating problem
because the vents are too restricted. First just bypass the NAT router
altogether and just connect your computer directly the DSL modem to make
sure all that works okay.