You should reset the router to factory settings.
Look in the router's manual for details, or do a search of the web for the router's model number for instructions on how to perform the factory reset. Most routers have a "reset" button, often it's very small and you will need to depress it with the point of a pen or another pointy tool.
After resetting the router, the encryption security on the wireless signal should be removed, and the default username and password should then be in place for the administration console (or "webpage settings"). Details should be provided in the router's manual.
Best of luck.