I don't know if this is the place to ask...
Not exactly... there is a networking section in this forums. :) You might get more help there, but anyway:
Sounds really strange, so let's make sure nothing easy happened:
Since it's very unlikely that something within normal usage killed a second router, too - have you checked your LAN cables carefully? I know from experience that they can be pretty sensible and a broken wire/damaged contact can work for some time until heat/the cat/your wife/coincidence makes the problem show up. The problem is: When it's got good contact again, your LAN card may need up to 5 minutes to re-establish the connection (if it does at all), so it looks broken all the time.
So have you already tried swapping that cord with another? I'm almost sure that this or related stuff could be your problem since your DSL modem (with the other cable) runs fine on the LAN as you reported.
What about the status LEDs on your router? Are they showing normal status on the WAN side and a broken LAN connection? Are they on at all?