If your iPad is unresponsive, you may have no alternative other than to perform a forced-restart - and then restart the transfer process.
If your iPad is unresponsive, connect your iPad to its Power Adapter and allow the iPad to charge. Then, while still connected to external power, try a forced-restart:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)
Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

When performing the transfer, keep both devices connected to their Power Adapter throughout the process - and this will keep the WiFi connections alive - and ensures that a low battery will not cause the transfer process to fail.