96 Cavalier won't shift past second, pops out at 30 - 40 mph

First, the check engine light is not on – no codes. If I’m in first, at the speed mentioned, the engine slows the car down – fast. In second, it pops out of gear. Acceleration is fine. Fluid and filter were changed after the problem started, no help. The front under the bumper found a curb when teaching daughter to drive seven years ago. Car was totaled when she was hit from behind and pushed into the car in front. Most of the damage was on the right side. Fixed it and it’s still a solid little car. Tried to find the computer to check for loose wires. Where is it and how do I get there? Also, what could be the cause of the problem and what’s the fix? I’m going crazy! So is daughter 'cause I have to use her car at times.