I would definately call your trans shop and check on the warranty. No reverse can be a few different problems and most require partial trans teardown to inspect. Does it just not work any time at all now or is only a cold or hot issue? If it comes and goes than it could be as simple as a seal problem that is fairly common on the reverse apply servo piston but if you dont have reverse under any circumstance it sounds more serious and could be a possible blown out channel plate gasket or broken band which means internal repair.
