Your linkage is coming loose on the transmission (usually). I had this problem with mine. Raise the hood and look on top of your transmission on the drivers side of the car. You will see a big black cable that comes out of cab and then loops back around and connects to the top of your transmission on the shifting linkage. You will see the that there is a nylon (plastic looking) connector that needs to be pushed back down. If it still comes loose then you need to get another shifting cable for your car. I found a used one on Ebay for $35.
If for some reason the cable hasn't come loose on the transmission then it has to have come loose on the other end in the shifter box. Dismantle the shifter box and check the connection there. One end or the other has come loose and you will most likely have to purchase a used shifter cable and replace it. I bought a used cable on ebay for $35.