OP:
Another "Fishrrman stupid question™", but have you tried powering OFF your ISP's modem/router, then repowering it and let the whole service "come up again" ...?
Then again, you might change out the ethernet cable between the iMac power supply and your modem/router -- or even just swap it "end-for-end" (assuming that's possible).
Another thing to try:
Open the network preference pane.
Click on "ethernet" (upper left), then click the "-" sign (bottom left) to REMOVE it.
Now, restart.
When you get restarted and back to the finder, open network pref pane again.
This time, click the "+" sign (bottom left) and ADD a new ethernet connection.
You never know, this might help...