With a more precise description it would be easier to help you.
However, here are a few things you can try. If they don't help, please provide more information.
First of all, try restarting and resetting your iPad.
Restart: turn off your iPad, wait for a few minutes, turn it on again.
Reset (reboot): press and hold Home and Power button until the Apple logo appears. Release both and let your device power up.
Turn off Wi-Fi, then on again (Settings - Wi-Fi).
Also there: tap the network that doesn't connect and choose "Forget this network" on the next page. You will have to set up this Wi-Fi connection once more.
If this doesn't help, reset your network settings in Settings - General - Reset. This will remove every information about your networks on your iPad. You'll have to set them up again afterwards.
If nothing I suggested solves your problem, maybe the router is the reason for it.