A few things to try:
-Reboot the router by unplugging the power, waiting 10 seconds, then plugging it back in and waiting another 20-30 seconds for the connection to reestablish.
-On your phone, go to Settings/Wireless & Networks/Wifi, select your network, and select "Forget." Then reconnect to it again.
-Adjust your router's security settings. You probably have to refer to your router's manual to learn how to do this. Try changing WPA to WEP or vice-versa, or the password format from TKIP to AES or vice-versa. As a last resort, try turning off all router security to see if that's blocking things.