Incorrect password;iphone unable to connect to LynksysRouterE900

I am unable to connect my iphone 4s to my Linksys Wi-Fi router (Model :E900). It was working fine until few months back. I am not sure what happened..but all of a sudden i am unable to connect my iphone to my router. I see the solution posted in this forum which tells me to update the security key for WEP. But, my security type is WPA/WPA2 which safer than WEP. I keep getting either "Incorrect password" in spite of entering right password or getting "unable to join the network". I did basic things like rebooting/resetting the router to factory settings, resetting network and all in my iphone etc., No use. Any help would be highly appreciated...

Thanks in advance.