My Outbox has disappeared

You only have an outbox if you have unsent mail. If all your mail has been sent there's no longer an outbox. It's a just a temporary box for mail that doesn't immediately get sent.

First, try starting with forced-restart of 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.

If the forced-restart does not resolve the problem, delete the email account from your iPad, again restart your iPad, then re-add the email account to your iPad:

Settings > Passwords & Accounts

Hello,

Can you please elaborate more on your question?

Writing an effective Apple Support Community post