If you have already tried any of these - just skip them. It is also possible that your router could need a firmware update so you could check that as well.
Go to settings>WiFi>Turn it off and then turn it on again.
Go to Settings>WiFi>Tap the blue arrow to the right of your network name>Forget this Network. Restart your iPad and see if you can join the network. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.
Go to Settings>General>Reset>Reset Network Settings. Restart the iPad and try to join again.
Go to Settings>WiFi>Tap the blue arrow next to your network name>Renew Lease. Restart the iPad and try to connect again.
This one is really an old one and I don't know if it still applies. Adjust the auto brightness on your iPad - Go To Settings>Brightness and Wallpaper - turn it off. See if you can connect.
People have different descriptions of rebooting but if you haven tried this - give it a shot. Reset the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons.