2001 Hyundai Elantra It frequently gets stuck in different

Quite frequently the car gets stuck in different gears, It started out as just 3rd, but now it gets stuck in all the gears. Usually putting it in reverse or park releases it, but now it has started going right back into the original stuck gear. It almost never shifts down into first, and when it tries to upshift, it usually groans then lurches real bad, its feels like when someone is learning to drive a manual trans. Originally precision tune auto said it might be a computer problem cause it does not register the code when the check engine light comes on, but then it was only a once in a while problem. Now its a constant problem. Also, I used to be able to manually shift it even when it stuck, but now I cant even do that. Is this possibly something simple, or is the transmission going kaput. Also, could this problem be caused by frequently over-due oil changes?