Auto focus problem
My first suggestion is to send it back if it is under warranty.
The grease probably got so thick, from the cold, that it caused too much resistance for the auto-focusing mechanism. You probably have damaged the focusing mechanism in the lens. You cannot repair this, yourself. I don't know how cold is too cold but it really shouldn't have done that.... Tamron makes cheap lenses. Try and exchange it for at least a Sigma.