My documents vanished from folders.

Try a forced restart of your iPad - as this will often clear an isolated software issue:

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:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. 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.

Presumably you will have either an iCloud or iTunes backup that has not been updated since the documents "disappeared' from your iPad. If so, you will be able to restore all your locally stored data from your backup:

Restore all content to iPad from a backup - Apple Support

If you don't have a backup, you may have irretrievably lost your locally stored files.

Any files listed as stored "On my iPad" are only stored locally on your iPad.

The entire point of creating a backup of your iPad - using iCloud, iTunes or elsewhere - is to create a safety-net. Should you accidentally delete a file that (a) cannot be otherwise recovered, or (b) something unexpected causes a loss of data, or (c) loss or theft of the device itself, you can restore your data from the backup.

If you value your data, always keep a backup - as this is your insurance policy. The unexpected can occur with any computer, Apple or otherwise - either through technical malfunction/failure or user error.