The best I can recommend is to just make sure that if the password is case sensitive that you enter it in precisely. As Corey suggested try changing the password to something really simple temporarily and try connecting using that password. If that works then it's likely that the cases aren't matching up.
another thing you can do is to tell your iPhone to "forget" the network. It's possible some weird setting got changed. To do this, go to the wifi in Preferences, touch the blue arrow next to your network. There you should see a "forget this network" button.