Turn off Wifi on your phone and turn off the Router for a few minutes. Turn it back on and turn Wifi back on your phone. If that doesn't work go to advanced wifi options on your phone by going to the wifi settings and press the menu button(left of Home Button) and pick Advanced, set the Regulatory domain to a low channel and Wifi policy to Never and see if that helps.
If it still doesn't work set a static IP to 192.168.0.9 and Gateway to your Routers gateway that can be found out clicking on start,run,typing cmd to pull up the command prompt and than typing ipconfig. Set Subnetmask to 255.255.255.0
If it still doesn't work maybe you may need to Replace the Router ?