Without the password it's not going to authenticate.
It'll be a bit of work, but connect to the router with a a computer connected by Ethernet cable (so you don't need the password). Change the wifi password (there are 2 - one for the router, one for wifi). Now go to all your devices that are connected by wifi and change the password in them to the new password you just put into the router's wifi section. Using that password, the S2 should connect also.