97 Hyun. Elantra - No 1st or reverse.

So for the past few months my car would slip out of first gear every once in a while, usually if I was slanted downhill a little and gave it too much gas. If I left off for a few seconds it would shift into gear.

Yesterday I turned in front of a car that was going faster than I thought and gunned it. It slipped out of gear, except when I let off the gas it wouldn't go back in. I pulled over and found that it would still go, but start out in what I think is first (very slowly, almost no power). I finally made it home and realized that it also won't go into reverse. Checked the transmission fluid and it isn't low, but it might actually be a little high.

Does it sound like the transmission will have to be rebuilt? Or could it be something else? Thanks for your help.