Gear will not go into reverse.

Sounds like it is not adjusted or centered on the connector rod
https://www.youtube.com/watch?v=xgyAPIqhkmY
May have issue inside car adjustment also. link below may apply
https://www.youtube.com/watch?v=GFa5maYbW8I

Alternative

Need to know the year, make, model of the vehicle and which engine and trans.

either you have lost the reversing gear or the gear shifting solenoid will not work.

Im having the exact same problem, word for word, with my 1995 3000gt! Anyone have any ideas on how to fix it please???

maybe your shift linkage has slipped out of normal position? If you can find Neutral in another gear position, it would tell you how far off it is.

4l60e or 4l80e transmission?

Could be your clutch or the cable from the shifter has a plastic piece that is easily broken that wont allow proper gear placement

Firstly is it slow for reverse, reverse clutch also engages in 3rd gear however reverse has a boosted circuit (more psi) therefore will engage rev but not 3rd but it is a little slow for reverse.
They do have a tendancy to pop a ball out of the High and reverse clutch giving this symptom.

????? manual or auto tranny if its an auto check the tranny