Car Transmission not engaging?

The van did not have its transmission oil changed for a long time. Yesterday while driving it was making a noise and the van was struggling to go forward then finally the car would not go neither reverse or forward (the engine is fine.. it starts with no problem). It all happened yesterday. Today when I moved the gear from Park to reverse or forward, nothing happened not even the little movements that cars make when changing gear. The transmission is not engaging/working. Will chaning the oil now help or it is too late? It is a Dodge Van Ram 2500, year 2000, RWD, automatic. Thank you.