Reconfiguring my router after a weird spontaneous malfunction...?

My roommate and I have been behind a D-Link 604 router for several weeks, and everything worked fine. Recently, we weren't able to connect to the internet and we discovered that after plugging each of our ethernet cables into the walljack individually, and connecting successfully, something is wrong with the router.

So my question is, how do I configure the router to allow access? I contacted our landlord, and they claim every should work fine, and they recommended reconfiguring the router. How do I do this?

I can't access the router configuration page because when I enter in the URL box the numbers that are found through Run > cmd > ipconfig, I'm prompted for a password that I can't get past (I never changed it, so the details are still user: admin, password: blank).

Any advice on how to fix this problem? Could it just be that the router is simply dead (it's 2-3 years old)?

Jam it back in, in the dark.

Well, resetting the router seemed to do the trick. Thanks for the (obvious) advice. What was the reason for this, offhand? I've never messed with my router. Are you just expected to reset it every once in a while?

There's nowhere I can't reach.