My Trash account is suddenly empty. How do I reinstate it so that I can retrieve messages that have been deleted?

There are settings in Mail Preferences > Accounts > Mailbox Behaviors to control when or if it automatically empties the Trash.

I believe that same setting is available on iOS, too, so you might check the settings on that device, too. I'm not sure there are, so I don't know where to tell you to look except in the account settings for that email account.

I presume you mean email, in which case the first thing to do is to login into your mail server via the web using Safari (or another Browser) and see if you can access the messages there.