i have 1984 cutlass supreme that all of sudden wont go into reverse
Without seeing the vehicle, that will be hard to diagnose. Have your mechanic look at for you.
If you still have the original (underpowered) turbo-hydramatic 200 or turbo-hydramatic 250 in the car that's the first sign of transmission failure. No reverse. My suggestion would be to replace the transmission with the better turbo- hydramatic 350. Unless you're trying to keep the car all original the 200 or 250 isn't worth fixing. HTH. -Jim