Not picking up my NAS

[#1]

Hi,
I have a NAS device plugged into one of the router Ethernet ports, and the NAS says it has a 100 Mbps connection. When I log into the router to get the IP, it does not recognise there is something attached to that Ethernet and therefore does not give me the internet IP address for it. I have tried changing the Ethernet plug and used different ports on the router but to no avail.

[#2]

Give us a few more clues
What kind of router? What kind of NAS?
Is it a new NAS or router? If not, has it been working OK in the past? If it has, what changed to trigger this problem?