I abbot turn my iPad on to back it up to iCloud.

If your iPad cannot be successfully powered-on - and the Passcode entered - it is too late to create a backup.

All locally stored data is encrypted; entering the Passcode unlocks the iPad security chip (the Secure Enclave) within which the encryption keys, necessary to access your data, are stored. If you can’t enter the Passcode, your data is beyond reach.

Your must create a backup before disaster strikes - not after.