Often when you shift into drive and the rpms drop but there is no power or movement you have lost an apply element. The forward bands are known to crack or break where the apply pin pushes on them but this will still give you manual 1 takeoff. If your input clutch piston came apart or the seal is torn you will lose fwd movement, or if the input clutch sprag went south you will lose fwd movement but the later is not common at all. There isnt a lot you can do outside of looking inside to find the problem. The fluid level is topped off correct?
