There are several things that you can do in order to solve this. Be sure that you are in range of the WiFi router. On your device, go to Settings -> WiFi and turn the WiFi on and off. Try to remove your WiFi network from the device and add it back after. If your WiFi options are dimmed, restart your device and make sure that the airplane mode isn't turned on. It is a good idea to update your device firmware to the latest version available.
You can also change your router WiFi security type to WPA, WPA2 Personal TKIP or AES. This could solve your problem.