If you have access to the router (which you should assuming it's yours), you can push the factory reset button on the rear and then set it back up.
open browser type 192.168.1.1. (general)
In the login menu type admin (mostly)
and your router password.
In router menu opened in ur browser in setup to to wireless, there u can change ur security key. Also in internet menu u can change ur password to connect to ur isp (it depends on who setup ur password, you or ur isp, either way u will have to call up your isp).
Resetting will change everything to factory settings. Do this only if u know how to setup everything from scratch again. Best is to call ur ISP