If you can find the wireless router, just plug a network cable into it. From there you'll get an IP address assigned, follow the steps above to get that address and see if you can access the router.
If no one is using it, maybe you could hard reset it. Usually you'll see a little button on the back that requires something like a pin or expanded paper clip to press. That will restore the factory settings and you should be back to Admin and Admin or Password for username and password.