Did you configure your router in the first place with your own settings and/or is the user and password admin/admin and do they work anymore?
If it is a wireless, it could be that someone else has been able to access it due to insecure settings and change them.
Althought that is unlikely, there is still a possibility for it. Reset the router to factory settings by holding the reset-button for 10 seconds and/or until the router reboots.