Hello,
Unfortunately, there is no way to retrieve messages deleted from the inbox. With help from an admin, you could check your institution's beta environment to see if the message was still there when your production environment was cloned to beta (which typically happens every Saturday), but that's about the only hope of getting it back that way. The default is that Inbox messages are forwarded out to an external email, so you could also check the default external email that your messages are likely forwarded out to and see if it's there.
