Try this.
These procedures are mostly for iPad 2 models that were having this issue, but may work for your iPad model, as well.
If you can't reactivate this iPad by simply re-inputting your Apple ID and password, then you need to proceed to the
next steps below.
If you have a cellular iPad model, try removing the SIM card from the SIM card tray, located on one of the long sides of the iPad ( you may need something small or pointy or the original SIM card tray tool that came with your Cellular service iPad to open the tray ), remove the SIM card, try either a simple restart of your iPad AND/OR try a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
Then, try re-activating your iPad without the SIM card in the tray.
If successful, return the SIM card back into the tray.
If no joy,
Leave the SIM card installed, first, and try to re-activate your iPad with it connected to a computer running iTunes.
To do this,
Connect the iPad to a computer with an active Internet connection and iTunes installed.
You are NOT trying to update or DO ANYTHING with the iPad through the iTunes app.
iTunes is just acting as a “helper” conduit app to help get your iPad to try and get activated.
Plug the iPad into the computer, wait for iTunes to “see”/recognize your iPad.
With the iPad still connected to the computer, go back to your iPad and try the Apple ID and password entries, again
to see if your iPad will activate while being connected to iTunes on the computer.
You should NOT be doing anything within iTunes itself.
If STILL no joy,
If your iPad won't activate with it connected to computer with iTunes with the SIM card in the tray, try all of this,
again, above, with the SIM card removed from the tray.
Best of Luck to You!