The same thing happened to me about a year ago on my '88 Reatta. Parked it, went in to work, came out 9 hours later, cranked her up, went to put in gear and the automatic shifter was loose and would not move to another gear. I had to have it towed ($120) to a great mechanic that had worked on my car previously and he said what happened was that a plastic clip that connected the cable to the shifter (my auto verbiage may be incorrect) broke and to get to it and replace it would mean pulling the transmission. So, about $1000 and a week later, the red monster was back on the road. That was a very expensive piece of plastic.
I hope yours is an easy fix and less expensive.
Tricia