First off, make sure the gimbal software is up to date in the app.
Does the gimbal immediately recover after leaving the smart shots? Have you had any hard landings? Any involuntary landings (ie; crashes)?
What I'm getting at is that it's possible (though not likely) that the gimbal control cable has become unseated from the socket on either the gimbal or the main board. I would pull the battery bay cover off, as well as the gimbal to reseat both connectors. If that doesn't solve the issue, you could try to do a factory reset which would require you to pair the solo with the controller again.