Car automatic shifter just broke down?

My car automatic shifter just broke down, the shifter moves but the dashboard stays on D so I left it on the handbreaks to stay. I tried pressing the breaks, moving the sterring wheel, turning it off and on, it all works well but it is still on D . Should I get the oil or transmission fluid checked? Or what else needs to be checked? I haven't kept up with it, any other suggestions would be helpfull so please help!