Transmission Issues

When we were driving all of a sudden the engine was just revving up. The transmission seemed like it wasn't engaging into the gears to move it. It finally went in and started to move the truck, but when I stopped at stop signs it didn't want to move. It wouldn't even move in reverse. I put it into low gear to get it to move. Then I would put it back into drive once I was moving. The O/D (overdrive) light was flashing when this stuff had happened. Then the light stopped flashing when I stopped the truck and shut the truck off, but the truck still wasn't moving right after I started it back up. The O/D button hasn't worked in a long time. Then all of a sudden everything started to work fine. I don't see anything leaking either. I don't want this happening again, what do you think this issue is? What do you think we should do?