There can be numerous reasons why your iPad might not be connecting to a WiFi network. You may have entered the wrong password for that network or may not have one for a preferred connection. There might also be a case where the profile you once connected with regularly might have ceased to exist. The network settings might also have changed on your iPad, which may also be one of the reasons.
If you cannot connect to a profile you once regularly connected with, you can try to fix this by selecting the “forget the network” option, then try reconnecting with it.