Restarting the iPad and your router would be good first steps.
After that, determine which security protocol your router is using. If WEP, that could be the source of the problem. You should use WPA2, if supported on your router. Use WPA, otherwise.
If your router is already using WPA or WPA2 or the problem still persists, renew the DHCP on the iPad. It has been a long time since I've looked at an iPad, but believe that is done by tapping Settings, tapping WiFi, tapping the symbol to the right of the SSID for the router. On the next screen, tap on Renew Lease.
Hope this helps
Tom