You need to reset the entire router to the factory default. On the back is a small hole labelled ‘reset’. Push a small wire or paper clip through the hole, and keep it pressed until the power light flashes. Then use the default password (admin) to access the router, there is no username.
Note that you will also need to configure all the other parameters that have been changed for your specific site, not just the password, once you have done this reset procedure.