Loss of numbers spreadsheets

You will either have manually created a full backup of your device using a PC (with iTunes installed) or a Mac (with Finder). An iCloud backup to your iCloud storage is automatic when your device is both connected to its charger and is locked - provided that iCloud Backup is enabled within your iCloud settings:

Settings > [Your Name / AppleID] > iCloud > iCloud Backup - is set to ON

Okay - not in iCloud - so we might assume that your data was stored locally. If a device restart did not recover access, we have to assume that the data was unexpectedly lost. As such, you should consider restoring your data from your iCloud or iTunes backup.

https://support.apple.com/en-us/HT204184

If you don’t have a backup of your data, then you’ve reached the end of the line...

First, begin by force-restarting your iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

Next, are your Numbers documents stored locally on your iPad, or in iCloud? If saved in iCloud, log-out from iCloud, then restart you iPad before logging-in again.

So as to verify that your documents are actually available within iCloud, you should log-in to iCloud from a Web Browser (any computer will do the job) and navigate to your Numbers documents.

If necessary, you can even launch the browser-version of the Numbers Application.

If your documents are visible, they’re in iCloud. If not visible from the browser, you might want to check the Recently Deleted area from your Browser. If not in iCloud, that might suggest that your documents were/are in your iPad local storage.