Do still you have devices connected to the router? If so, it might be possible to go into wifi settings on those devices and view the password. The password will typically just be shown as dots, but I think I've seen it where there's a "show password" checkbox you can click to reveal the password.
Alternatively, you can log into your router's web page and just change the security key there.
Edit: My bad, misread the OP. Thought you were looking for the wifi password. Did you ever change your router login info? It may still be the default login, which you can probably google for your router model. Often something like username: admin, password: password.