Why? Too early to say that

If it was caused by fluid leak, it will be easy to fix.

A while ago I had the same problem. While I was driving through a small ditch my car's bottomed hit the ground but I kept driving and then parked it in a parking lot.By the time I got my things done, I started the car and drove away. However I only drove it for about 20 yards before the transmission started slipping and the car would not move forward anymore. I checked the transmission fluid level and found out all the fluid was gone. It turned out that the transmission oil pan was pinched a small hole when the bottom hit the ground. I took off the oil pan and spent $5 to have the hole welded by a shop. Put it back on and added transmission fluid, the car ran again.