2001 Pontiac Bonneville 2001 Pontiac Bonneville won't chang

Recently my mothers' Bonneville won't go into gear. Ecspecially reverse, we have it parked in the garage. I believe when she puts it in a forward gear she can inch it forward, neutral might seem to work. (Checking on that today) but can't get it to go into reverse. When we shift gears it acknowledges it on the dash, but there is no sound of changin anything. So we plan on putting it in neutral today and backing it up, and will see if any gears work. If anyone knows what the problem is, it would be greatly appreciated, the sooner the better to get it over to a mechanic. (The reason why I'm on here is the one primary mechanic my mom has a preference to is telling her she needs a whole new transmission. I need to make sure they aren't tryin to screw her over.) Thank you very much for your time reading this.
some comments i've read is bad tranny pump, bad torque converter/sensor, and trans overhaul.