all of my email has disappeared

Emails you see in Mail app are actually stored on an email server and mirrored to the device.

With most of email providers you don’t need an app such as Mail to access emails; they give you an access to what is called webmail tool to view the account and your emails; it’s a website your provider let you sign in to your account to check your emails. See if using that website you can see emails you are missing in Mail app on your iPhone.

If they are missing on that website too, then that is the reason you don’t have access to emails in Mail app. Your email provider would need to help your to recover missing emails.

Can you access your emails through webmail?

Have you checked via webmail access if your missing emails are still stored on the server?

If they are missing on the server and in Mail app, contact with you email tech support to find what server side data recovery options they have.

If emails are missing in Mail app only, then check if filter messages button in lower left corner of the screen isn’t toggled on. Otherwise, remove the email account, then reboot your iPhone before adding the account back.