No warning signs ...car wont drive in any gear but reverse! Help!!

So my car has been driving great no warning signs at all as far as a failing transmission. When I put the car into drive or any of the other gears I don't hear it change gear.. but it does rev fine and the car itself sounds okay..it just doesn't move forward. It moves in Reverse fine there are no check engine lights on. I'm just afraid I'm going to go to a mechanic and they're going to tell me I need a new transmission. Could it be a solenoid or some other easier fix? 2007 Pontiac G6 any advice is appreciated and I've read around and all the other posts related and that some people have had success removing the pan however when I called a mechanic he said when you remove the pan all there is is a filter.