Unable to change screen time passcode in iPad - keyboard / number pad doesn't show

I would try to put it on charge for at least half an hour, then, keeping it plugged in, try a "forced" restart of your iPad, using these instructions:

• On an iPad with a Home button: Press and hold the power button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: In rapid sequence, press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the power button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).

If the issue persists, bearing in mind these instructions:

These are the minimum requirements to reset the ScreenTime passcode:

• The device must run iOS/iPadOS 13.4 or later;
• You must know the Apple ID & Password that were used to set up that ScreenTime Passcode;
• If you are part (member) of Family Sharing, ask the Organizer.

If you meet ALL above requirements, these are the instructions:--> If you forgot your Screen Time passcode – Apple Support

Shouldn't be this case, you need to erase that device following these instructions (even if related to the passcode to lock/unlock device's screen):--> If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

The bad news is that you cannot restore data from a backup, since the ScreenTime passcode would come back in.

If the issue still persists, contact Apple Support, using this link:--> Contact Apple for support and service

Regards

Giulio